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PREFACE ^paragraph 25

     * I am more afraid in the present treatise of occasional
     misconception in respect of some expressions which I have
     chosen with the greatest care in order that the notion to
     which they point may not be missed. Thus, in the table of
     categories of the Practical reason under the title of
     Modality, the Permitted, and forbidden (in a practical
     objective point of view, possible and impossible) have
     almost the same meaning in common language as the next
     category, duty and contrary to duty. Here, however, the
     former means what coincides with, or contradicts, a merely
     possible practical precept (for example, the solution of all
     problems of geometry and mechanics); the latter, what is
     similarly related to a law actually present in the reason;
     and this distinction is not quite foreign even to common
     language, although somewhat unusual. For example, it is
     forbidden to an orator, as such, to forge new words or
     constructions; in a certain degree this is permitted to a
     poet; in neither case is there any question of duty. For if
     anyone chooses to forfeit his reputation as an orator, no
     one can prevent him. We have here only to do with the
     distinction of imperatives into problematical, assertorial,
     and apodeictic. Similarly in the note in which I have pared
     the moral ideas of practical perfection in different
     philosophical schools, I have distinguished the idea of
     wisdom from that of holiness, although I have stated that
     essentially and objectively they are the same. But in that
     place I understand by the former only that wisdom to which
     man (the Stoic) lays claim; therefore I take it subjectively
     as an attribute alleged to belong to man. (Perhaps the
     expression virtue, with which also the Stoic made great
     show, would better mark the characteristic of his school.)
     The expression of a postulate of pure practical reason might
     give most occasion to misapprehension in case the reader
     confounded it with the signification of the postulates in
     pure mathematics, which carry apodeictic certainty with
     them. These, however, postulate the possibility of an
     action, the object of which has been previously recognized a
     priori in theory as possible, and that with perfect
     certainty. But the former postulates the possibility of an
     object itself (God and the immortality of the soul) from
     apodeictic practical laws, and therefore only for the
     purposes of a practical reason. This certainty of the
     postulated possibility then is not at all theoretic, and
     consequently not apodeictic; that is to say, it is not a
     known necessity as regards the object, but a necessary
     supposition as regards the subject, necessary for the
     obedience to its objective but practical laws. It is,
     therefore, merely a necessary hypothesis. I could find no
     better expression for this rational necessity, which is
     subjective, but yet true and unconditional.

In this manner, then, the a priori principles of two faculties of the mind, the faculty of cognition and that of desire, would be found and determined as to the conditions, extent, and limits of their use, and thus a sure foundation be paid for a scientific system of philosophy, both theoretic and practical.

Nothing worse could happen to these labours than that anyone should make the unexpected discovery that there neither is, nor can be, any a priori knowledge at all. But there is no danger of this. This would be the same thing as if one sought to prove by reason that there is no reason. For we only say that we know something by reason, when we are conscious that we could have known it, even if it had not been given to us in experience; hence rational knowledge and knowledge a priori are one and the same. It is a clear contradiction to try to extract necessity from a principle of experience (ex pumice aquam), and to try by this to give a judgement true universality (without which there is no rational inference, not even inference from analogy, which is at least a presumed universality and objective necessity). To substitute subjective necessity, that is, custom, for objective, which exists only in a priori judgements, is to deny to reason the power of judging about the object, i.e., of knowing it, and what belongs to it. It implies, for example, that we must not say of something which often or always follows a certain antecedent state that we can conclude from this to that (for this would imply objective necessity and the notion of an a priori connexion), but only that we may expect similar cases (just as animals do), that is that we reject the notion of cause altogether as false and a mere delusion. As to attempting to remedy this want of objective and consequently universal validity by saying that we can see no ground for attributing any other sort of knowledge to other rational beings, if this reasoning were valid, our ignorance would do more for the enlargement of our knowledge than all our meditation. For, then, on this very ground that we have no knowledge of any other rational beings besides man, we should have a right to suppose them to be of the same nature as we know ourselves to be: that is, we should really know them. I omit to mention that universal assent does not prove the objective validity of a judgement (i.e., its validity as a cognition), and although this universal assent should accidentally happen, it could furnish no proof of agreement with the object; on the contrary, it is the objective validity which alone constitutes the basis of a necessary universal consent.

PREFACE ^paragraph 30

Hume would be quite satisfied with this system of universal empiricism, for, as is well known, he desired nothing more than that, instead of ascribing any objective meaning to the necessity in the concept of cause, a merely subjective one should be assumed, viz., custom, in order to deny that reason could judge about God, freedom, and immortality; and if once his principles were granted, he was certainly well able to deduce his conclusions therefrom, with all logical coherence. But even Hume did not make his empiricism so universal as to include mathematics. He holds the principles of mathematics to be analytical; and if his were correct, they would certainly be apodeictic also: but we could not infer from this that reason has the faculty of forming apodeictic judgements in philosophy also- that is to say, those which are synthetical judgements, like the judgement of causality. But if we adopt a universal empiricism, then mathematics will be included.

Now if this science is in contradiction with a reason that admits only empirical principles, as it inevitably is in the antinomy in which mathematics prove the infinite divisibility of space, which empiricism cannot admit; then the greatest possible evidence of demonstration is in manifest contradiction with the alleged conclusions from experience, and we are driven to ask, like Cheselden's blind patient, "Which deceives me, sight or touch?" (for empiricism is based on a necessity felt, rationalism on a necessity seen). And thus universal empiricism reveals itself as absolute scepticism. It is erroneous to attribute this in such an unqualified sense to Hume, * since he left at least one certain touchstone (which can only be found in a priori principles), although experience consists not only of feelings, but also of judgements.