an idealistic position: cognition needs not only the activity of the understanding and its forms but also the manifold of intuitions and the forms of intuitions. As the example shows, in order to think of a line, the subject has to “draw” it in space. That is to say, categories are not the only sufficient and necessary conditions of cognition; space and time are also needed.
Moreover, it is important also to remark that the ‘I think’ is not to be viewed from an empirical-psychological level but, instead, as a transcendental principle necessary for the justification of the possibility of experience. Kant’s statement of the necessity of each representation to be guided by the ‘I think’ does not imply that the condition of the objectivity must be a clear empirical consciousness, self-consciousness:
“Now it does not matter here whether this representation be clear (empirical consciousness) or obscure, even whether it be actual; but the possibility of the logical form of all cognition necessarily rests on the relationship to this apperception as a faculty.” (KrV A117)79←66 | 67→
There is a huge difference between consciousness as self-knowledge and as a priori condition of the unity of the experience: the first, given and determined in the internal sense, is a representation; while the second, the ‘I think’: “it is the consciousness of the spontaneity of the thinking, but it does not reveal in itself an actual determined existence. This latter existence will follow from the diversity given through sensibility.” (de Vleeschauwer 1934–37, II, p. 228)80.
Differently from the empirical ‘I’, the ‘I think’ does not facilitate the knowledge of the self, but knowledge of the fact that a self is given:
“In the transcendental synthesis of the manifold of representations in general, on the contrary, hence in the synthetical original unity of apperception, I am conscious of myself not as I appear to myself, nor as I am in myself, but only that I am.” (KrV B157)81
But this reference to the ‘I think’ can also lead to ambiguities: to be aware of being, at this level of inquiry, does not mean to be conscious of one’s own personal existence, but of the fact that there is an experience. In Kant’s view, the ‘I’ of the ‘I think’ does not refer to the personal identity, but it is merely a function, a necessary unity to justify the unity of the experience.
Now, what are, then, the conditions of the unity of experience? Which faculties are implied in the process of cognition? In Kant’s words, the conditions of possibility of experience can be summarised as follows:
“There are, however, three original sources (capacities or faculties of the soul), which contain the conditions of the possibility of all experience, and cannot themselves be derived from any other faculty of the mind, namely sense, imagination, and apperception. On these are grounded 1) the synopsis of the manifold a priori through sense; 2) the synthesis of this manifold through the imagination; finally 3) the unity of this synthesis through original apperception.” (KrV A94)82←67 | 68→
It is not sufficient to claim that sensible impressions are given in sensibility according to space and time, but it is necessary to state that they are reproduced by the imagination (that allows the representation of an object although without a present intuition) and that they are all united in the understanding in order to make possible the experience of a unitary object and not of separated impressions, in which the consciousness would dissolve itself.
Although Kant distinguishes three different sources of consciousness, the synthesis’ process is unique83:
“In such a way it is proved that the synthesis of apprehension, which is empirical, must necessarily be in agreement with the synthesis of apperception, which is intellectual and contained in the category entirely a priori. It is one and the same spontaneity, that, there under the name of imagination, and here under the name of understanding, brings combination into the manifold of intuition.” (KrV B162)84
And in the section “On the ground of the distinction of all objects in general into phaenomena and noumena”:
“With us understanding and sensibility can determine an object only in combination. If we separate them, we have intuitions without concepts, or concepts without intuitions, but in either case representations that we cannot relate to any determinate object.” (KrV A258/B314)85
In this way, according to Kant’s view, the synthetic a priori judgements find a first (although not completed) explanation through the Transcendental Deduction: the subjective conditions of the understanding have an objective value, because they are the basis of the possibility of the phenomena’s constitution; the a priori understanding anticipates the form of the possible experience in general, thus determining the field of possibility of an object of experience in general.←68 | 69→
But so far, the transcendental inquiry is not completed. Although Kant has explained that pure concepts are necessary conditions of objective experience, he does so only “from the side of the understanding” (KrV B162), that is to say, categories are not enough to provide a complete explanation of how experience is possible, which is one of the aims of the Critique (KrV B20). As Kant states:
“[…] categories contain the grounds of the possibility of experience in general from the side of the understanding. But more about how they make experience possible, and which principles of its possibility they yield in their application to appearances, will be taught in the following chapter on the transcendental use of the power of judgement.”(KrV B167)86
Kant has to face a crucial question: can the forms of the understanding be applied to the matter of experience and can understanding and sensibility, despite their fundamental heterogeneity actually work together? Both of these faculties are provided with pure forms. The objective validity of the forms of sensibility does not need justification, because it is only through them that objects are given in experience. As Kant puts it:
“In the case of the concepts of space and time, we were able above to make comprehensible with little effort how these, as a priori cognitions, must nevertheless necessarily relate to objects, and made possible a synthetic cognition of them independent of all experience. For since an object can appear to us only by means of such pure forms of sensibility, i.e. be an object of empirical intuition, space and time are thus pure intuitions that contain a priori the conditions of the possibility of objects as appearances, and the synthesis in them has objective validity.” (KrV A89/B121)87
Categories, on the contrary, need a justification of their objective validity and Kant provides it in the “Deduction of the Pure Concepts of the Understanding”. ←69 | 70→However, the possibility of cognition is not sufficiently explained. The two faculties and their forms must work together, but they are profoundly different: sensibility is merely a receptive faculty, while understanding is active; space and time do not need a transcendental deduction, while categories do because they are conditions of thinking, i.e. they could be regarded as mere subjective rules without necessary relation to objects (KrV A90–91/B123–124). As Kant states:
“Thus a difficulty is revealed here that we did not encounter in the field of sensibility, namely how subjective conditions of thinking should have objective validity, i.e. yield conditions of the possibility of all cognition of objects; for appearances can certainly be given in intuition without functions of the understanding. I take, e.g., the concept of cause, which signifies a particular kind of synthesis, in which given something A