The Prolegomena were published in 1783, two years after the Critique of Pure Reason, Kant's groundbreaking work. They were conceived both as an answer to the first objections and as a second attempt to present "critical philosophy" in a less "diffuse" and "obscure" manner. Apart from providing a better overview of the whole project, the Prolegomena sketchily indicate the ultimate objective of Kant's philosophy: to establish a metaphysical system of a priori principles that by its rigor would match the stringency of scientific knowledge. In contrast to the Critique of Pure Reason, the Prolegomena start from the existing forms of knowledge and then analytically sift through them until the very principles of Metaphysics are reached. After highlighting the historical and the systematic context of his enterprise, Kant expounds his new doctrine of synthetic a priori judgments which are supposed to produce objectively valid knowledge for any future Metaphysics (First Philosophy). He first demonstrates how pure theoretical sciences make use of these judgments, then discusses the ideas of reason and finally, in the conclusion, he answers the question how Metaphysics can become a science.

What is the meaning of the full title: Prolegomena to Any Future Metaphysics (that will be able to present itself as a science)?

The keyword in the title is the Greek term prolegomenon (singular) which denotes a prefatory essay or a foreword to something that is supposed to follow. Accordingly, the prolegomena (plural) are a kind of introductory examinations designed for "preparatory exercises". By spelling out what "we have to do to make a science actual if it is possible" these exercises should prepare the emergence of the only viable Metaphysics - the "scientific" one .  

Note: In view of this ambition, the Prolegomena are not just "a more readable version" of the Critique of Pure Reason, as they are commonly characterized. More than that - they offer both probably the best introduction to Critical (Kantian) Philosophy and develop some new points. In fact, very few books formulate so many complex ideas within such a limited space - the circumstance that should excuse the size of this outline as well.

Analogy:

A comparison with Descartes' Meditations on First Philosophy reveals a structural parallel:
Cartesian Meditative Initiation into First Philosophy = Kantian Preliminary Exercises into any Future Metaphysics
(First Philosophy = Metaphysics).

The Meditations lay ground for the existing metaphysical and scientific knowledge. The Prolegomena are more radical in their preparatory stance as they clear the ground for an absolutely new science of Metaphysics.

The New:

For Future Teachers

For Anatomists of Reason

In order to avoid possible misunderstandings Kant explicitly states that the Prolegomena are not written (a) for "the use of mere learners" or (b) for those "scholarly men" who identify philosophy with the history of philosophy.

(a) The Prolegomena do not offer a "ready made science". Instead of propounding well rounded doctrines the book provides a method for the "discovery" of the metaphysical science. One learns doctrines, not the method. Consequently, it is designed for future teachers who will develop the content by applying the method rather than for present apprentices. But even teachers cannot expect a fully "systematic exposition" of the long searched science.

Note: There is no discrepancy between being designed only for teachers and being a "preliminary exercise". Teachers in philosophy do not teach doctrines (what to think) but the way how to think. Strictly speaking, one cannot teach philosophy, because there is none which could be accepted as an established science. Consequently, the Prolegomena cannot be the subject of any mechanical, scholastic exposition. Yet in making use of this "introduction", we can teach to philosophize, which is perhaps an even greater achievement than a "finished philosophy", although at this stage the whole process does not go beyond a mere exercise.

(b) The historians of philosophy tend to track down new views to the old tenets in accordance with the rule that "nothing could be said which… has not been said before". Since philosophy has a very long history, no wonder that it provides an abundance of possible similarities, echoes and points of comparison for any new position. But the content of the Prolegomena is not a piece of historical knowledge. It is something new. The time for the historians of philosophy will come in due course after those who "draw from the fountain of reason" (anatomists and critics) finish their work. But right now Kant strenuously affirms the legitimacy of the new against the analogies which may dissolve it into "the old sayings of the past". In contrast to the historical approach, he requires openness for fresh insights, not their reduction to something previously  known.

Note: This plea for novelty is very indicative for the spirit of the Enlightenment and Modernity in general. Kant is aware that he himself belongs to the Modern Age and he fully understands the nature of the Modern Age (striving for the new, scientific rigor and critically secured foundations). It is no accident, therefore, that the word "modern" occurs self-reflectively in the Prolegomena (here, for instance, Kant makes a distinction between "Ancient" and "Modern" philosophy). The ironical remarks about historical analogies are probably directed toward J.G.H. Feder, an eclectic contemporary of Kant, who had compiled a review of the Critique garnered with many artful historical labels. All this could be as well an allusion to Eberhard, another contemporary, who has regarded the new philosophy of Kant just as a reformulation of Leibniz's views.

In order to be able to proceed without prejudices, Kant suggests to "pause a moment" and consider "all that has been done as though undone". (Descartes similarly decided to withhold assent to all existing beliefs as if they were false.) By taking a fresh look at the things we can clearly grasp the current state of affairs and ask the very first question, the question of possibility, which otherwise would not find receptive minds.

Note: This strategy is reminiscent of the first Meditation: To start anew, let us go back to the foundations while suspending our assent to everything derived from them. Kant wants us to disregard the whole previous history of  Metaphysics and to reexamine its very idea and principles before we can reach the verdict about its fate.

Is "such a thing as metaphysics… possible at all"?

Cartesian parallels to this question would be: "Is indubitability possible at all? And what are its conditions of possibility?" But Descartes could have raised these questions only rhetorically. Despite all his doubts, he did not have any dilemmas about the possibility of Metaphysics. After all, it has gone through a very long development from Aristotle up to the time of Rationalism.
Kant himself points to this rich tradition stretching "since the origin of Metaphysics so far as we know its history". Is not his own question about the possibility then very odd and superfluous? Does it make sense to ask whether something "be possible" when that something already exists? All the more so as Kant admits that Metaphysics is not only a past historical reality, but something which possesses present "actuality".

How can Kant then ask a question which plainly casts doubt on the existence of Metaphysics? He himself acknowledges that "the question whether metaphysics be possible presupposes a doubt as to its actuality". The solution to this apparent contradiction is that Kant is not speaking about the Metaphysics as it has been and as it is. This Metaphysics is possible by virtue of the fact that it exists. But this Metaphysics is the realm of "illusionary knowledge" and thus not viable. In raising the question of possibility, Kant had in mind another Metaphysics, a new, "future Metaphysics". This new Metaphysics is currently just an idea, but to make it reality is very desirable given the present state of the discipline. It is therefore possible to confront in a non-contradictory manner the factually existing Metaphysics with the norm what Metaphysics should be. What should it be? A science ("if it be science"). Thus the true meaning of the preliminary question could be rendered as:
"Whether Metaphysics is possible as a science"? 

This matching with science is a novelty even for Kant who has always valued scientific rigor. Truly, he claimed to have been "in love with Metaphysics" since his early years (under influence of the Wolffian Metaphysics), but in the Critique he seemed resigned over the idea that Metaphysics could ever be raised to the level of the sciences. Obviously, in the Prolegomena he changed his stance on Metaphysics .

Note: The origins of the identification between philosophy and science hark back to the time of Aristotle, but the Ancient concept of science was much broader (episteme as the knowledge of truth in general) than the Modern idea of science (methodical and experimentally verifiable knowledge). In the Modern Age these attributes of science started to be regarded as the conditions and factors of its advancement. After Kant, philosophy has been constantly subjected to the demands to demonstrate the same level of objectivity and methodological solidity as "exact sciences". This pressure accounts for the prevailing tendency (from the 19th century onward) to define philosophy as a science. At the same time, this tendency reveals that the relationship between philosophy and science has become precarious. Having lost the status of the "queen of sciences" philosophy now must decide whether it wants to demarcate itself definitely from the sciences or is it rather inclined to be their "foundational discipline" - an ambition which arises from the Kantian project. It is noteworthy that in Kant's eyes Newton replaces Galileo as the scientific role model (the time of Descartes has definitely gone).

Non-Science:

Kant begins with a blunt assertion that Metaphysics does not exist as a science. It is now only a "so called science",  a science by name, a "supposed jewel". What really exists under the name of Metaphysics is just a caricature of wisdom and science. Being a non-science or phony treasure is tantamount to virtual nonexistence in terms of valid knowledge, which accounts for an evidently counterfactual statement: "There is as yet no such thing as metaphysics." This circumstance both justifies and renders meaningful the question of possibility. Were Metaphysics as a science already "in existence", the question whether it be possible at all would be superfluous ("useless"). One would be able then simply to refer people to it and ask them to study it as the truth. "Here is metaphysics; learn it and it will convince you irresistibly and irrevocably of its truth." The only questions that would then make sense would be "How is it possible?" and "How does reason come to attain it?" But as of yet, there is no book of Metaphysics which in philosophy enjoys the status comparable to Euclid's Elements in Geometry. Hence we cannot point to any present work and say: "This is metaphysics, here you may find the noblest objects of this science, the knowledge of a highest being and of a future existence, proved from pure reason."

Note: This sentence could be a direct reference to the Meditations whose subtitle promises a demonstration of "the existence of God and the immortality of soul".

Current State: 

What we encounter in current metaphysical books are very "doubtful assertions" that directly undermine each other ("in all ages one metaphysics contradicted another, either in their assertions or their proofs"). Viewed from the vantage-point of its history, Metaphysics appears as "the battlefield of perpetual controversies". The constant clashing of its systems has entirely compromised the ambition of Metaphysics to attain "lasting acceptance". Hence the history of Metaphysics does not offer much support for its admittance into the ranks of respected disciplines. Traditional Metaphysics has been giving promises of furnishing profound insights ("wisdom incarnate"), but has never fulfilled its announcements. Nonetheless, it continues to keep the audience in perpetual suspense by repeatedly nourishing hopes that the most "noble" questions would be eventually answered. Time and again, however, these hopes were dashed with the emergence of every new system. New systems resemble desert mirages that vanish once we approach them. "So metaphysics, floated to the surfaces, like foam, which dissolved the moment it is scooped off." Repeated failures to receive viable answers to metaphysical questions have finally destroyed human trust into Metaphysics ("its votaries having melted away"). Moreover, the perpetual succession of dogmatic metaphysical systems has contributed immensely to the "appearance of skepticism", which is a "mental attitude" of utter cognitive desperation.
Note: Even though Kant has been receptive for skeptical arguments, he was not less opposed to skepticism than Descartes - on the contrary, his style of philosophy is clearly constructive. The ultimate goal is clearly to build (erect) new "towers" of Metaphysics (this is Kant's equivalent for the Cartesian metaphor of "edifice").

Defects of Metaphysics:

When compared with the sciences Metaphysics suffers from the following shortcomings:

Decision:

Note: Kant is deeply dissatisfied with the present state of Metaphysics, but he does not want to abolish it entirely. In the Prolegomena he recognizes not only the legitimacy of Metaphysics as a "natural disposition", but goes a step further and envisions its upcoming scientific renovation. To be able to subsist Metaphysics only needs to be articulated in a manner befitting to a science.

Polarity:

We cannot any longer tolerate the schizoid stretch between "the importance of the knowledge so much needed" and the growing suspicions regarding "all knowledge which we believe we possess". Something needs to be done to get out of this deadlock. In heeding this call Kant wants to avoid both the uncritical dogmatism which overestimates the abilities of human reason and the hypercritical skepticism in which "reason treats itself with a such violence" (i.e., denies its own capacities). The former, despite great promises, "teaches us nothing", the latter "does not even promise us anything" and ultimately leaves everyone with nothing.

Radical Reform:

Examining the Foundations:

Prior to the "final verdict" about the fate of Metaphysics Kant wants to see whether traditional Metaphysics could be perhaps renovated ("a new birth") according to "a new plan". The problem of Metaphysics cannot be resolved without rebuilding. This means Metaphysics has to transform itself into a science. To accomplish this it needs to begin anew. Piecemeal patching does not help much. On the other hand, a radical renewal cannot start properly without laying down good foundations for the construction.

Fortunately, human constructive drive pushes anyway into the same direction of clearing the ground even if that requires some demolition: "Human reason so delights building that it has several times built up a tower and then razed it to see how the foundation was laid." This inspection is more important (critical) than destruction or construction. The examination of the foundations is beneficial in a twofold sense: it can inform us about the current state of the structure or it can teach us how to perform a better construction. As to the first, Kant contends that in the process of science building we need at one point to ask whether the current status of a particular science is in accord with the conditions of its possibility. As to the latter, the preliminary question whether Metaphysics is possible, could be viewed as preliminary to another beginning. Once it is answered positively with regard to a more constructive work, the renovation of Metaphysics can be safely kicked off.

Obstacles:

(1) The renovation plan should be executed as early as possible: the more delayed the realization of the need for change, the more difficult would it be to carry out the reform. Regretfully, the awareness that Metaphysics needs reconstruction comes pretty late in the history of philosophy.

Note: Kant agrees (with Epicure) that "it is never too late to become     reasonable and wise", but he disagrees that it does not matter when  we gain this insight. Had it come centuries ago it would have been easier to correct many mistakes and avoid resistance.   

(2) Spreading doubts about old and accustomed views is embarrassing and cannot count on favorable reception. Doubts about Metaphysics offend those who identify with the present Metaphysics. On the other hand, the rising doubt will ran into the wall of intellectual dogmatism: the traditionalists will look at the doubter condescendingly ("in the proud consciousness of their ancient possessions") whereas the contemporaries will overlook the novelty and ignore the whole project as "something (already) seen before". 

Prolegomena, from the Greek prolegomenon = something said in advance; in plural, the word denotes introductory discourses.

Metaphysics, from the Greek meta ta physika = after (the books of) physics; 1. the title given to the collection of Aristotle's lecture notes on First Philosophy (dealing with the first principles of being and with being as such); 2. traditionally, the study of the most general and abstract principles, the study of the universe in its entirety, the study of the ultimate reality, the study of the supernatural (occult, mystical, spiritual, immaterial). 3. Along with Descartes, Kant envisions a different task for Metaphysics: to become the study of the first principles of human knowledge. He uses the word in three different ways though: (a) Metaphysics is the realm of (illusionary) knowledge beyond any possible experience - traditional Metaphysics; (b) Metaphysics denotes all a priori knowledge and its exposition, thus including both the traditional and the reformed Metaphysics  = in this sense Metaphysics is synonymous with Philosophy in general; (c) Metaphysics is the system of a priori principles which rests on a thorough analysis of human cognitive faculties = in this narrow sense, conceived as a science, Metaphysics is almost indistinguishable from Transcendental Philosophy.

Preliminary, from the Latin prae + limen = coming before the threshold; prefatory, introductory, preparatory examination preceding the main exposition.

Dogmatism, from the Greek dogma = opinion, belief, tenet; a philosophical position ascribed by Kant to the traditional Metaphysics which goes beyond possible experience; dogmatism rushes to explain the universe by deducing inferences from a set of starting principles without previously defining the limits of human knowledge. 

"Since the Essays of Locke and Leibniz, or rather since the origin of metaphysics…nothing has ever happened which could have been more decisive to its fate than the attack made upon it by David Hume."

In the first part of this statement Kant refers to the following writings:

Note: Kant both follows and changes the traditional use of the terms "reason" and "understanding". In ancient philosophy reason was conceived as the faculty of direct intuitive knowledge of principles (nous) whereas the understanding was credited with deductive calculative reasoning (dianoia). Early modern philosophers forget this distinction and use both terms synonymously. In accord with this practice Kant had initially defined both reason and understanding as "the capacity to judge". However, later on he started to distinguish between the two by describing only the understanding as the faculty of mediate judgment (i.e. the faculty of concepts). Reason, on the other hand, became the faculty of inferences and ideas transcending experience. Thus reason turned out to be a higher faculty in charge for the unification of thought. However, in the historical part of the Prolegomena Kant still uses reason and pure understanding interchangeably as general terms for the faculty of concepts.  

Negative Relevance:

In the second part of the above quoted statement on his predecessors Kant makes a very flattering appraisal of Hume's work: nothing was more "decisive" for the fate of Metaphysics than Hume's critical remarks. With regard to the recognized Hume's influence on Kant, this evaluation is understandable, but in this particular context it sounds pretty odd. How an outright (and devastating) rebuttal of Metaphysics could be regarded as the most important event in the history of Metaphysics since its inception? Is this just an ironical remark from somebody who in fact rejects Metaphysics himself (the best contribution to Metaphysics is to do away with it) or a more profound insight into the nature of Hume's position than anything standard textbooks say about the matter? This question is all the more in order since Kant right after this line qualifies his evaluation of Hume by saying that the "acute man", with the exception of some "sparks", did not throw "any light on this species of knowledge" ( a typical backhand compliment). How was it then possible to achieve such a decisive impact on Metaphysics without even clarifying its notion? Obviously Kant sees the relevance of Hume's work in its "negative use", that is, in the attack on Metaphysics. Hume did not contribute to Metaphysics directly and constructively, but he "challenged (metaphysical) reason" in a way that had an immediate bearing on the status of Metaphysics.

Challenge to Reason:

How did Hume pose that challenge? By disputing that reason "gives birth" (is the "birthplace") to the concept of cause and effect (and its physical counterpart: force and action). Hume 'pressured' the reason to answer "by what right she* thinks anything could be so constituted that if that thing be posited something else also must be necessarily posited." In other words, what is the basis for the belief that if A is the cause of B, then B necessarily follows whenever A occurs. The answer was: none whatsoever. (The fact that the streets are wet does not necessarilly mean that it rained; the fact that it rained does not necessarilly entail the consequence that the streets are wet - something could have prevented that outcome, trivial or extraordinary.)

*Note: In the Paul Carus translation as revised by Lewis White Black "reason" is referred to as "she" both to avoid potential confusion with the word "cause" and because it is feminine in German (die Vernunft).

"He inferred that reason was altogether deluded with reference to this concept." Both the necessary and a priori character of causality disappeared upon examination. What does this have to do with Metaphysics? By denying any role of reason in the formation of the concept of causation Hume has undermined Metaphysics in two ways:

(a) Since reason is the main organ of metaphysical thinking (speculation), Metaphysics is automatically rendered impossible if the categories of reason are denounced void.

(b) Ever since Aristotle Metaphysics has been the study of the causes. Deprived of the concept of causation, Metaphysics cannot derive anything from the first principles nor can it speculatively track back the existing effects to their first causes (thus, for instance, no cosmological proof of God's existence would be possible). The whole edifice of reality becomes loose while metaphysical systems get reduced to "the castles in sand". "In plain language, this means that there is not and cannot be such thing as metaphysics at all."

Without causation it is not possible to establish any rational linkage between objects that could be conceptually articulated or accounted for. Strictly speaking, we can make then only judgments about our individual impressions or recite tautologies like A = A. Everything else would belong to the realm of arbitrary and illusionary knowledge (the "play of words").

Note: Despite his mistrust toward rational Metaphysics, Hume retained the title "Metaphysics" and even went so far to attach to it "great value". Obviously, he could not help looking at his "destructive science of metaphysics" as a kind of Metaphysics - after all he was aware that his own statements on Metaphysics were not mere impressions or relations of ideas. But the admissible Metaphysics consisted, for him, only in "the negative side" of his enterprise, which was bent toward "moderation of extravagant claims of speculative reason, and the complete settlement of the many endless and troublesome controversies that mislead mankind". Kant approves this aspect of Hume's work, but he reproaches that Hume has entirely overlooked "the positive injury" which is inflicted on human knowledge "if reason be deprived of its most important prospects". (The "injury" affects not only metaphysics but the sciences and even common sense, although Hume explicitly targeted only metaphysical assumption that everything happens by reason and things are causally connected.)  

Hume was interested in finding out whether there is any necessary connectedness between different events and objects in the world. That is to say, whether things are causally connected.
By using Kantian language one can elucidate Hume's concerns by means of the following questions: 

  Hume answered all three questions with an unambiguous "No".

Hume was not satisfied with the empiricist assumption that causal relation is empirically observable for he realized that necessity, as an essential component of the relation, could not be detected experientially. On the other hand, he was not able to find any rational justification for it since it does not follow from any logical principle either. This was an impasse that required explanation. In response, Hume contended that the connection between cause and effect is not a priori, is not objective, is not necessary and is not universal. "He demonstrated irrefutably that it was perfectly impossible for reason to think a priori and by means of concepts such a combination, for it implies necessity." This necessity is not included in the concept. On the other hand, we do not see any necessity when observing events in the actual world. All "the pretended a priori combinations" are "nothing but common experiences" of successive events. What we find there is just a physical proximity of two events in space and a close sequence in time.

From the above considerations Hume drew the following conclusions:

This is to say, the metaphysical supposition that there is a principle of causation connecting different things and events in the world is absolutely unfounded. Hence, the connection of cause and effect is not conceptual at all. Consequently, Metaphysics is an illusion; as a futile endeavor it is at best just an "agreeable pastime". 

Origin:	Experience
Formation:	Imagination
Operation:	Association by similarity and proximity
Result:	Habit mistaken for objective necessity

Though harsh and disquieting, Hume's contentions are extremely important and, consequently, needed to be taken seriously into account ("deserved the concentrated attention of  the brighter spirits of the day"). However, Hume's "valuable" critique of Metaphysics was "misconstrued" and finally rejected by his "opponents". Who were they? The staunchest defenders of Metaphysics? Surprisingly, not. They were the Scottish philosophers of common sense (Reid, Oswald, Beattie, Priestley). Why would common sense philosophers criticize an attack on Metaphysics? Certainly not because they were lovers of Metaphysical sallies. Why, then? Out of common interest with it ("interests intimately interwoven"). Common sense assumes the applicability of the principle of causation to account for everyday events. It is a common sense belief that when I turn a glass of water upside down I "cause" its content to spill out, that is to say, common sense operates under the assumption that causation articulates our experience.
In addition, one can suppose that these philosophers might have been appalled by skeptical and atheistic implications of Hume's critique and so reacted out of the commonsense allegiance to the current religious beliefs.

Note: Kant had in mind the following works of the Scottish philosophers: Thomas Reid, An Inquiry into the Human Mind on the Principles of Common Sense, 1764; James Oswald, An Appeal to Common Sense in behalf of Religion, 1766; James Beattie, An Essay on the Nature and Immutability of Truth in opposition to Sophistry and Skepticism, 1770; Joseph Pristley (the famous scientist), Letters to a philosophical Unbeliever, 1780.

Common Sense:

Basis of Rationality

Good Use

Perverted Sense

Restrictions:

Although appreciative of the plain (right) reason's achievements (it is a 'good sense'), Kant vehemently combats demands for popularity and appeal to common sense in matters of metaphysical investigations. He views this appeal as a convenient way to avoid the consequences of Hume's analysis. For Kant, common sense is not a correction of Hume's extremes, but a slippage below the level of Hume's insights. Therefore he occasions a brief but penetrating excursus on common sense. In it Kant singles out both the qualities and the limitations of commonsense approach to philosophical issues.  

              A. Achievements:

1. Common sense is a "great gift of God" (natural aspect). Endowed with "plain common sense" humans have secured a realm of rationality between brutality and insanity.

2. Common sense is one "of the subtile discoveries of modern times" (social aspect). The appeal to common sense is a modern democratic right which establishes equality among all normal disputants regardless of their real rank. If it comes only to common sense then "the most superficial ranter can safely enter the lists with the most thorough thinker and hold his own" (which is a great political achievement).

              B. Conditions:   

1. An appeal to common sense should be backed by "some well considered and reasonable thoughts and words". It should not be  performed in an "oracular" way as if common sense provided a solution to all philosophical problems before they are even posed. (In philosophy we are not permitted to invoke common sense as a "magical wand" or simply assume that everything is self-evident when there are many lingering questions.)

2. A reference to common sense is sometimes everything that is available. It is in order to invoke common sense when we do not possess scientific knowledge or other insights ("when insight and science fail"). (However, if some insight is still available the appeal to common sense turns into an inappropriate or premature extension of its use.)  

             C. Shortcomings:

1. Substantially the authority of common sense is based on "the opinion of the multitude". Philosophers should be very reserved toward it ("ashamed") because its credentials are utterly suspect ("depends merely upon rumor"). To invoke common sense indiscriminately means therefore to take a recourse to something that lacks any critical rigor.

2. The appeal to common sense could be just a "subterfuge" to avoid an open-minded discussion. Sometimes disputants cling slavishly to common sense because of habit or inability to consider alternative solutions. In that case the use of common sense comes down to a renouncement of "all pure speculative knowledge" (theoretical cognition) for some mundane purposes of "practical life".

             D. Restraining Order:

(1) Common sense is in its true element "in judgments which apply immediately to experience" (trivial, plain situations), but should not be permitted "to speculate or to make decisions in maters of speculation" ("when we judge universally from mere concepts"). Common sense observations cannot sustain universal and necessary knowledge. 
(2) When common sense transgresses into the realm of "pure concepts" and wants to judge there, it ceases to be the "right", "plain" or "sound sense" and becomes "sick". Instead of securing a safe ground for reasoning it transforms something more complex into something overly simplified. This is the reason why it needs to be constantly kept "in check" and restrained from damaging incursions into the matters of philosophy.
(3) Therefore an appeal to common sense is absurd whenever we need knowledge of rules in abstracto, independent of experience. "Chisels and hammers may suffice to work a piece of wood, but for etching we require an etcher's needle."

Misunderstanding:

Whatever the case with common sense, the Scottish philosophers were "taking for granted that what Hume doubted, and demonstrating with zeal and often with impudence that what he never thought of doubting". Not surprisingly, their unexpected defense of Metaphysics proved to be useless simply because it has entirely "missed the main point" of Hume's problem. The Scottish philosophers mistook the "question of origin" for the "question of need". In other words, they have confounded the issue of status with that of potential usefulness. But Hume did not deny that the concept of causation is or might be very useful ("indispensable need of using it"); he only questioned its usual cognitive justification. In that sense, of course, the question of origin became the question of validity.
Kant points that the major failure of the Scottish common sense philosophers was that they have not "penetrated very deeply into the nature of reason, so far as it is concerned with pure thinking". (Indicatively, Kant reproaches the same failure to Hume.) They simply resorted to common sense (causation is "evident") without realizing that for a viable rebuttal of Hume (who certainly possessed an abundance of common sense) they would have needed more than that: "a critical reason". Kant therefore demands a more scrupulous and more dialectical approach in assessing Hume's findings than what common sense can accomplish.

Common Sense, from the Latin sensus communis = the sense shared by all people who possess normal perceptual and intellectual abilities; everyday, ordinary understanding; beliefs or conclusions which would be accepted by all normal individuals.

Speculation, from the Latin speculum = mirror; 1. traditionally, the way of theoretical thinking which causally relates visible, immediately present effects to their directly invisible, but in these effects indirectly "reflected" (mirrored) ultimate causes; 2. for Kant, speculation is the type of knowledge dealing with the most universal and abstract principles that are furthest away from sensory knowledge ("judgments based on pure concepts"); synonymous with philosophy in general, = pure thinking, pure reason, speculative understanding. 3. popular meaning of speculation: an action (with stocks or finances) which is very uncertain but aimed at quick gains.

Causation, from the Latin causa = thing, that which is responsible for something; 1. the idea that when A occurs and B invariably follows, then A represents the cause of B, whereas B, being produced or changed, is the effect of A; 2. in general, the belief that any change or event is produced by something else, without which that thing would not have happened.

A priori, from the Latin a priori (parte) = from the preceding (side); originally, a priori was an adverb denoting the way of cognitive approach; 1. the expression was initially used to denote the reasoning leading from ground to consequence or from the earlier part of the cause-effect relation; 2. after Descartes the term is used mostly adjectivally for a type of knowledge independent of experience; thus it started to designate everything we know as universal and necessary truth prior to or independent of experience (concepts, propositions, knowledge in general); of course, some experience may be necessary to acquire the concepts that formulate an a priori proposition.

A posteriori, from the Latin a posteriori (parte) = from the posterior (side); 1. in contradistinction to a priori, a posteriori originally denoted any reasoning from consequence to ground; 2. later it came in use for all knowledge based on experience, i.e., for all knowledge contingent upon empirical and probable facts.

Understanding, (German: Verstand) as the etymology of the word suggests, it is the faculty of recognizing that what "stands among" (under); in Kant, a spontaneous faculty of judging which transforms immediate intuitions into objects of thought by means of representations of representations (concepts); it is the faculty which secures the unity of immediate representations (appearances) according to rules; these rules are a priori categories stemming from pure understanding.

Reason,  (German: Vernunft) the faculty of inferences and universal principles; it pursues the unity of the rules of understanding under principles; beyond that, it strives toward "absolute totality" in the synthesis of conditions for all possible experience thereby transcending every given experience (transcendent ideas are pure concepts of reason). 

Dogmatic Slumber:

Correction:

Hume should be given a credit for the discovery that causal connections do not reside in the objects but are produced in and by the observers of the objects. Even though he ascribed their production to imagination, he was the first to realize that so many concepts we have been taking for granted actually stem from subjective principles that surpass the immediacy of impressions. His particular merit lies in raising the question of the origin and validity of  the problem of causation. Hume asked which faculty is responsible for the concept of causation (senses, imagination or reason) and, after pointing to the collaboration of the first two, he raised the question of its objective valididity. Prior to Hume "no one had hesitated to use the concepts  without investigating the basis of their objective validity." Everybody just assumed that future (conclusions) will be like the past (conclusions) simply because future always has been like the past. But Hume has shown that this assumption begs the question which is precisely how do we know that the same effects will follow in the future (An Enquiry Concerning Human Understanding, sec. 4). By posing the unasked question of validity and by unveiling the subjective origin of causal nexus Hume has awakened Kant from his "dogmatic slumber".

Note:  The phrase "the recollection of Hume" (that "interrupted" Kant's "dogmatic slumber") should not be understood literally, as if it suddenly occurred to Kant that he had read An Enquiry Concerning Human Understanding 12 years or so earlier and then (many years later) in a fit of a surprising illumination realized its relevance (as is suggested by the translator's footnote). There is in fact nothing about this passage that suggests that Kant is referring to a "recollection" at all. The word "recollection" is a translational error. Here Kant simply uses the German word "recollection" (Erinnerung) in its archaic meaning of admonition or warning (Ermahnung, Warnung). Several translators have mistaken it for the commoner modern meaning "recollection". The issue was thoroughly clarified by Michael Scarpitti (see http://www.hkbu.edu.hk/~ppp/texts/VERSCH.html). In terms of chronology, this means that Kant is not referring to 1783 as the year when the "awakening" occurred, but much earlier, possibly to the years around 1770 when he in fact had effectively started to revise his previous uncritical views under the influence of Hume's skepticism (the process that led to his "Copernican revolution"). Scarpitti believes that the reference could be pushed back even further (to the 1760s or even earlier) based on Kant's "Dreams of a Spirit-Seer".[There is independent support for this hypothesis among some Kant scholars.]

A. If some Scottish philosophers have missed the main point in Hume's reasoning this does not mean that his reasoning was correct in itself. Hume did not solve his own problem. He regarded rational principles and concepts as idiosyncratic patterns of our nature, that is, as only psychologically conditioned. By accepting such a reductionist view he accounted at best only for the experiential side of the problem of causation.

B. Hume's fault was twofold: he did not address the question of causation in its entirety and did not examine pure reason (no more than his opponents). In order to answer properly Hume's questions one needs to focus on "the whole of his problem" (the faculty of pure reason) of which causation is only "a part" (there are some other categories of reason as well). As Hume failed to do that, his conclusions about the impossibility of a priori knowledge turned out to be "hasty and mistaken". Before examining the role of reason in the whole process he jumped to the conclusion that the principle of causation is just a "custom-bred habit" thus delivering both the objective validity and experience to sheer habitual associations. His conclusions proved therefore premature, unwarranted and harmful.

C. In general, Hume's would be solution was not felicitous at all because it led to a depreciation of rational knowledge. Instead of ending in an outright dismissal of the metaphysical science, "a happier solution" could have "resulted in a complete reform of the science". That is to say, instead of relegated Metaphysics to a non-binding "past-time", Hume could and should have tried to secure its foundations by recognizing the real impact of reason on our metaphysical concepts. "Thus the acute man would have been led into considerations which must needs be similar to those that now occupy us…"

Causal Rationalism:

Kant agrees with empiricism that we can identify particular causes only through repeated perceptions. But he rejects the surmise that the concept of cause is derived from observation in the same way as our knowledge of what is the cause of some particular event. He agrees with Hume that the concept of causation is not empirical. But he contends that the concept of causation was not derived indirectly from experience by virtue of imagination, as Hume conjectured. For Kant, the concept of causation as other a priori concepts originated from reason itself ("pure understanding"). Yes, causal connection is  subjective by its origin, but no  less objectively valid for that matter. Judgments of causation are rendered possible and objectively valid when the objects of experience are "subsumed under those pure concepts of the understanding" (categories). Complex synthesis, not mere association is the fundamental principle of our knowledge. Once we determine correctly the synthetical origin of causation, we can easily ("as a matter course") define "the conditions of the use and the sphere of its valid application".

Two Goals:

This task was accomplished in the Critique of Pure Reason (1781). This book is the work in which Kant for the first time discussed "the pure faculty of reason in its whole compass and bounds". Consequently, the inquiry had a double objective: 

By including the concept of causation in the context of other a priori forms of understanding Kant offered not only a solution to that particular problem but also "a pilot and safe principles of navigation" in the whole realm of pure reason ("with respect to the whole faculty of pure reason"). This achievement definitely separated Kant from Hume who did not envision such a "formal science" ending instead in skepticism (he "ran his ship ashore, for safety's sake"). Safety was common concern for both Hume and Kant, but the latter responded to the need of securing our knowledge constructively by providing a system of navigation (concepts and catgories) along with the pilot (pure reason).

The system of pure reason had to be mapped in its entirety before any part of it could have been broached. The plan for a general critique of pure reason clearly presupposes the priority of the whole. Here Kant makes an analogy with the constitution of an organism: the whole consists of particular parts, but "the end of each member can only be deduced from the full conception of the whole" (otherwise we'll be committing the fallacy of composition). Thus, "we can do nothing without first determining the position of each part and its relation to the rest."

The accomplished "general critique of pure reason" is the precondition not only for a successful renovation of Metaphysics but also for a preliminary exercise toward that goal. "Critique as a science must first be established as complete and perfect before we can think of letting Metaphysics appear on the scene or even have the most distant hope of attaining it." Critique alone contains the plan and the means to "establish Metaphysics as a science" and it performs its role by exhibiting "the whole stock of a priori concepts", deduced and proved in their position and function. Once the critique is completed "down to the most minute elements of pure reason", the ground rules for new Metaphysics will be set. The "system of metaphysics" could be built based on the principles formulated in the Critique.

Kant has masterfully executed his Critique in the "widest extent" and in the most extensive way. But the work came out "long-winded", "dry" and "obscure" - the fact which probably explains why Kant's solution did not fare much better before the public than Hume's original challenge (A Treatise on Human Nature). Kant anticipates that the Critique will be misjudged because it will be misunderstood, and it will be misunderstood because it is not tailored to fit "ordinary notions".
Kant was aware of these defects and particularly that the Critique of Pure Reason lacked the intuitive clarity. But he rejected all reproaches about the "want of popularity, entertainment and facility". He maintained that this last complaint was unfounded simply because the possibility of "indispensable knowledge" can be established only "by the strictest rules of a scholastic precision".

Note: This is again an allusion to Feder who had complained about the "scholastic apparatus" of the Critique. In responding to Feder's complaint Kant implicitly makes the following distinction: 1. Scholastic precision = fitting the scientific character of the subject; 2. Popular presentation = serving the needs of those who want to understand the subject in concrete images. For Kant, 1 is the condition of 2. But 2 is not necessary at all; it "may follow", but it can precede only in a "sketchy" manner. 

But Kant dismisses both suggestions about the possible place and function of the Prolegomena. A short presentation like the Prolegomena would not have been much useful prior to the Critique of Pure Reason because, as "a mere sketch", it could hardly have been more than a proclamation of intent (something that befits "boastful minds"). A subsequent popularization was a possibility but Kant has never been interested in any digested presentation of his work. What is then the true purpose of the Prolegomena?

The task of the Prolegomena is to make sure that the absolute novelty of the project ("perfectly new science") is duly acknowledged and not misjudged as the old content just differently wrapped ("fancy pattern with new titles", "systematic garment" for "antiquated knowledge"). Beyond this, the Prolegomena should "enable us to grasp the whole" better and "to examine in detail the chief points of importance in the science" (of Metaphysics). With regard to the Critique of Pure Reason, the Prolegomena thus entertains a twofold objective: (a) to improve the exposition of the content by explaining better some murky points, and (b) to remove growing misunderstandings and respond to accumulated objections. Thus it is very helpful as a sequel despite the fact that it does not provide the final proof for the correctness of the arguments. But its main ambition is to tackle the same area from a different angle and to do that by applying a different approach.

Note: In terms of the content the central portion of the Prolegomena stands in the following relation to the Critique: the first part, dealing with the possibility of pure mathematics, corresponds to the Transcendental Aesthetic, the second part, which discusses the conditions of the possibility of pure natural Science, is a counterpart of the Transcendental Analytic, whereas the third part of the "main Question", on Metaphysics as a natural disposition and the ideas of reason, follows the Transcendental Dialectic of the Critique. The last main section, on the conditions of the possibility of Metaphysics in general, corresponds to the Transcendental Doctrine of Metaphysics.    

The Critique was carried out in the synthetical manner "in order that the science may present all its articulations, as the structure of a peculiar cognitive faculty". Everything was there deduced and accordingly proved from starting elements as they have been elaborated by critical reason. "In the Critique of Pure Reason I have treated this question (= whether Metaphysics is possible?) synthetically, by making inquiries into pure reason itself and endeavoring in this source to determine the elements as well as the laws of its pure use according to principles."

By contrast, the Prolegomena, being exempt from obligation to furnish an extensive exposition, proceed analytically. "The Prolegomena must therefore rest upon something already known as trustworthy, from which we can set out with confidence and ascend to sources as yet unknown, the discovery of which will not only explain to us what we know but exhibit a sphere of many cognitions which all spring from the same source. The method of prolegomena, especially of those designed as preparation for future metaphysics, is consequently analytical."

The first approach, going from the principles toward more complex statements, is progressive, while the latter, moving backwards from the givens toward the principles, is regressive ("this signifies only that we start from what is sought, as if it were given, and ascend to the only conditions under which it is possible").

Note: Strictly speaking, Kant proceeds analytically only in paragraphs 6 through 23. After that, given the dialectical nature of ideas of reason, he reverts to the synthetic method. The regressive (analytic) method should not be confused with the analytic content consisting of tautological statements. Kant acknowledges that the former usage of the term 'analytical' departs from the notion of analyticity (equal to a priori) as we are now accustomed to. But the former usage is not Kant's innovating. Prior to him, Christian Wolff already used the expression in that sense. In fact, the origins of this notion of analysis conceived as the succession backwards from the agreed conclusion are ancient (cf. Aristotle and Pappus).
J.P. Sartre obviously borrowed this terminology from Kant while discussing the progressive and the regressive method in his Critique of Dialectical Reason.

Kant dismisses all remaining reproaches regarding the lack of clarity. Those who may find even the Prolegomena obscure are coldly advised to "apply their talents to other subjects". They may hopefully prosper in the fields of "exact" (mathematical) and "deep sciences" (physics) which are "more closely allied to the empirical". In view of this, the residual obscurity (which is more often an excuse for the lack of intellectual effort than a cover for incompetence) proves even helpful because it could repel the aggressive ignorant (a worse species for Kant than the indolent).

Those, on the other hand, who are capable of mastering "abstract concepts", are faced with a very narrow choice: if they want to construct "a system of metaphysics" they must satisfy the demands made here, either by adopting Kant's solution or by thoroughly refuting it and substituting with another. With a full awareness of the discriminative nature of his work Kant says without false modesty: "To evade it is impossible." Kant himself was convinced that the suggested solution, given the difficulty of the subject, was not at all so complicated and abstruse.  

Deduction, from the Latin deducere = lead from; a reasoning which draws a valid inference from the (more general) premises; Kant uses the term in the sense of old jurisprudence, according to which a deduction establishes the legal, as opposed to the factual grounds for action; by the same token, his deduction is the justification of the application of the categories to objects, i.e., an explanation of why these categories necessarily apply to all possible experience. 

Category, from the Greek kategoria = accusation, i.e., attribution of certain properties, kind of predication; 1. Aristotle: a category designates the way in which a predicate is attributed to the subject; he lists ten most general kinds of predicates that exhaust all possible ways of attributing something to something; 2. Kant: there are twelve fundamental concepts by which  intellect structures our understanding and experience of the world; they are derived from the traditional division of judgments according to four regards: quantity, quality, relation and modality.

Critique, from the Greek krisis = discerning, judging, deciding; 1. originally, neutral examination of the subject in order to make necessary distinctions about the nature of the subject; 2. in the popular usage, sorting out negative characteristics of the subject (fault finding); 3. Kant revives the original meaning but applies it to one privileged subject: critique does not deal with persons, books and doctrines but with the faculty of reason; it defines the conditions of possibility and the legitimate use of reason insofar as it strives for knowledge independent of experience in positing the conditions of sound thinking critique establishes the standards and sets the boundaries of knowledge; thus the critique of reason, far from being the rejection of reason, provides in fact the enlightenment of reason; at the same time it makes decision about the possibility of Metaphysics.