First Postulate.—That which agrees, in intuition and in concepts, with the formal conditions of experience is possible.
Second Postulate.—That which is connected with the material conditions of experience (that is, with sensation) is actual.
Third Postulate.—That which is determined, in its connection with the actual, according to universal conditions of experience is (that is, exists as) necessary.
In this section Kant maintains that when the Critical standpoint is accepted, possibility, actuality and necessity can only be defined in terms of the conditions which render sense-experience possible. In other words, the Critical position, that all truth, even that of a priori principles, is merely de facto, involves acceptance of the view that the actual reduces to the experienced, and that only by reference to the actual as thus given can possibility and necessity be defined. The Leibnizian view that possibility is capable of being defined independently of the actual, and antecedently to all knowledge of it, must be rejected.
An analysis of the text can be profitably made only after a detailed examination of Kant’s general argument; and to that task we may at once apply ourselves. The section affords further illustration of the perverting influence of Kant’s architectonic, as well as of the insidious manner in which the older rationalism continued to pervert his thinking in his less watchful moments.
First Postulate.—In the opening paragraphs Kant uses (as it would seem without consciousness of so doing) the term possibility in two very different senses.[1258] When the possible is distinguished from the actual and the necessary, it acquires the meaning defined in this first Postulate; it is “that which agrees with the formal conditions of experience.” But it is also employed in a much narrower sense to signify that which can have “objective reality, i.e. transcendental truth.”[1259] The possibility of the objectively real rests upon fulfilment of a threefold condition: (1) that it agree with the formal conditions of experience; (2) that it stand in connection with the material of the sensuous conditions of experience; and (3) that it follow with necessity upon some preceding state in accordance with the principle of causality, and so form part of a necessitated order of nature. In other words, it must be causally necessitated in order to be empirically actual; and only the empirically actual is genuinely possible. Such is also the meaning that usually attaches to the term possible in the other sections of the Critique. A ‘possible experience’ is one that can become actual when the specific conditions, all of which must themselves be possible, are fulfilled. An experience which is not capable of being actual has no right to be described even as possible. As a term applicable to the objectively real, the possible is not wider than the actual, but coextensive with it. As Kant himself remarks, those terms refer exclusively to differences in the subjective attitude of the apprehending mind.
This ambiguity in the term ‘possibility’ has caused a corresponding ambiguity in Kant’s employment of the term ‘actuality.’ It leads him to endeavour to define the actual, not in its connection with the conditions of possibility, but in distinction from them. The possible having been defined (in the first Postulate) solely in terms of the formal factors of experience, he proceeds to characterise the actual in a similarly one-sided fashion, exclusively in terms of the material element of given sensation. Doubtless the element of sensation must play a prominent part in enabling us to decide what is or is not actually existent, but no definition which omits to take account of relational factors can be an adequate expression of Critical teaching. Indeed, we only require to substitute the words ‘sensuously given’ for ‘actual’ in Kant’s definition of the third Postulate (i.e. of the necessary) in order to obtain a correct statement of the true Critical view of actual existence: it is “that which is determined in its connection with the sensuously given according to universal conditions of experience.” For Kant the actual and the necessary, objectively viewed, coincide. Necessity is for the human mind always merely de facto; and nothing can be objectively actual that is not causally determined. As the empirically possible cannot, in its objective reference, be wider than the empirically necessary, one and the same definition adequately covers all three terms alike. While the distinctions between them will, of course, remain, they will be applicable, not to objects, but only to the subjective conditions of experience in so far as these may vary from one individual to another. Experiences capable of being actual for one individual may be merely possible for another. And what is merely actual to one observer may by others be comprehended in its necessitating connections. The terms will not denote differences in the real, but only variations in the cognitive attitude of the individual.
Thus in professing to show that the three Postulates are transcendental principles, Kant does less than justice to his own teaching. For though both here and in the opening sections of the chapter[1260] he speaks of them in this manner, i.e. as being conditions alike of ordinary and of scientific experience, he has himself admitted in so many words the inappropriateness of such a description.
“The principles of modality are nothing more than explanations [not, it may be noted, proofs] of the concepts of possibility, actuality and necessity, in their empirical use, and are therefore at the same time restrictions of all the categories to this merely empirical use, ruling out and forbidding their transcendental [= transcendent] employment.”[1261]
That is to say, these so-called principles are not really principles; they merely embody explanatory statements designed to render the preceding results more definite, and especially to guard against the illegitimate meanings which the Leibnizian metaphysics had attached to certain of the terms involved.