At this point one must explain the three domains of a priori forms.

First, is the region of sense knowledge. This is the field of receptivity by which ‘we are affected by the objects’ (A19); ‘objects given to us by means of sensibility, and it alone yields us intuitions’ (ibid). Now, that intuition is called empirical ‘which is in relation to the object through sensation’ (A20) and which is therefore based on received impressions. By way of contrast, a pure intuition is that ‘in which there is nothing that belongs to sensation’ (ibid.) ‘it belongs to pure form of sensibility’ (ibid.) and is actuated in the mind a priori, even without a given object. Correspondingly transcendental aesthetics is ‘the science of all principles of a priori sensibility’ (A21); but these are two ‘namely, space and time’ (A22), and they are the conditions for the possibility of the a priori synthetic judgements of mathematics. Space is more particularly the form of the external sense faculties i.e. ‘the subjective condition of sensibility, under which alone outer intuition is possible for us’ (A26). Time on the other hand, is ‘the form of inner sense, that is, of the intuition of ourselves and of our inner state’ (A33). Again, time ‘is the formal a priori condition of all appearances whatsoever’ (A34) because the external representations also belong ‘in themselves , as determinations of the mind, to our inner state’ (A34). To space and time is ascribed ’empirical reality’ (A35), that is, an objective validity, as the conditions alone that enable Man to perceive objects; in the same way they have a ‘transcendental ideality’ (A36) in so far as they are merely conditions of our sensibility and cannot in any way, be ascribed to ‘things as they are in themselves’ (A39). Thus, it is true they make a priori synthetic suppositions possible, but these apply to objects ‘only insofar as objects are viewed as appearances, and do not represent things are they are in themselves’ (A39).

Second, is the area of reason. As spontaneity in the production concepts, reason co-operates with the sense faculties, which are ‘receptivity for impressions’ (A50); through these impressions the object is ‘given’ whereas through reason it is ‘thought’ (A50). Knowledge can arise ‘through their union’ for thoughts with concepts are blind and intuitions with concepts are empty. Everything depends on the pure concepts; in these, ‘there is no mingling of sensation’ (A51). Thus transcendental analytics consists in the dissection of the faculty of understanding itself insofar as, in this faculty the pure concepts have been located and prepared in an a priori way. (A65-66). It is thus that the synthetic a priori judgements of natural science are explained with regard to their possibility. Hence the question is how to seek pure concepts on the basis of a single principle and how to determine in an a priori manner their systematic completeness. (A67). Because reason reaches only to the mediate knowledge of an object, or judgements, it constitutes a ‘a faculty of judgement’ (A 69). The elementary forms of judgement are outlined in it; from the point of view of the mere form of understanding they can be classified ‘under four heads, each of which contains three moments’ (A70). Coordinated in these are the 12 pure concepts of understanding (e.g. substance, causality etc.) which apply ‘a priori to objects of intuition in general ‘ (A79) as the true primary concepts of the pure understanding they are called ‘the categories’ (A80).

To the process of educing them is linked transcendental deduction, which shows that the categories are ‘conditions of the possibility of experience and are therefore valid a priori for all objects of experience’ (B161). However, objects must be understood not as things in themselves, but as appearances in space and time that are determined by the categories (B168-169). Consequently there can be no a priori knowledge, except of objects of possible experience (B166). Subject to the same limitations are the principles that teach reason (being for potency for judging) how, ‘to apply to appearances the concepts of understanding’ (A132). To these principle belongs the principle of succession in time in accordance with the law of causality: all alterations take place in the conformity with the law of the connection of cause and effect (A189, B232)

Third, is the study of the intellect, which is the concern of transcendental dialectics. Here the synthetic a priori judgements of metaphysics are examined as to their possibility. The question is not how they are possible, but simply whether they are possible. Kant’s answer is that they are not. Here his concern is to show the ‘transcendental illusion’ (A297); it is a natural and inevitable illusion (A298), and thus it has been able to lead metaphysics astray until now. The critique must be applied to ‘transcendental principles’ that in contrast to immanent principles, lead Man to go beyond the limits of possible experience (A295). At the basis of these principles one finds ‘transcendental ideas’ designed by reason in an a priori and necessary fashion:

‘No object adequate to the transcendental idea can ever be found within experience..for they view all knowledge gained in experience as being determined by an absolute totality of conditions..by an absolute whole.’ (A327)

From the different ways of drawing conclusions, Kant gathers that there are three and only three ideas, namely:

1. The soul as the absolute unconditioned unity of the thinking subject
2. The world as the absolute unity of a series of conditions of appearance
3. God as the absolute unity of the condition of all objects of thought in general (A334)

These ideas never allow of any constitutive employment as supplying certain objects. In other words, the objects that are outlined in these ideas are never recognised through them; for ideas that go as far as the thing-in-itself remain empty, because the intellectual intuition that complements them and reaches out into the realm of the thing in itself has not been given to man. On the other hand these ideas have an indispensably necessary, regulative employment insofar as they put before the intellect ‘the form of a whole of knowledge’ and in this way can determine a priori for every part of its position and relative to other parts (A644-645). This delimitation of their use is opposed by the sophistications that rise from the very nature of reason and claim to form a bridge from ideas to their corresponding objects. The four paralogisms intend to proceed from the transcendental concept of the subject to a science concerning the nature of thinking being. Just as rational psychology is impossible, so also is rational cosmology (A340, 345, 408). When the latter is attempted as the absolute totality of condition for any given appearance. In like fashion there can be no rational theology (A631) for all three kinds of proof for the existence of God are inconclusive, since the physico-theological proof rests upon the cosmological proof and the cosmological proof upon the ontological proof (A630), which itself suffers from an illegitimate transition from the realm of concepts to the realm of reality. What is left of God is only the transcendental ideal, as the concept of all reality and the complete determination of things without requiring that this reality be objectively given and itself a thing.