The transcendental concept of reason is directed towards absolute totality in the synthesis of conditions, and never terminates save in what is absolutely unconditioned.
It is the unconditioned which contains the ground of the
synthesis of the conditioned.
The pure concepts of reason are the unconditioned of:
The Categorical synthesis of a subject.
The Hypothetical synthesis of the members of a series.
The Disjunctive synthesis of the parts of a system.
Idea: a necessary concept of reason to which no corresponding object can be given in sense experience.
Thus the pure concepts of reason are transcendental ideas.
They are not arbitrary, they are imposed by the nature of reason itself.
They are transcendent and overstep the limit of experience.
We might say that the absolute whole of appearances is only an idea because we can never represent it in image.
The idea of practical reason is the condition of all practical employment of reason.
In the practical use of reason, reason is exercising causality, as actually bringing about that which its concept contains.
Reason, considered as the faculty of a certain logical form of knowledge, is the faculty of inferring, i.e. judging mediately.
The ascending series of syllogisms (toward the unconditioned) must be regarded by reason as completed and given in its totality.
Reason is indifferent to the descending series as to how far it goes and whether a totality is possible at all.
A transcendental dialectic has to contain, completely a priori, the origin of certain modes of knowledge derived from pure reason as well as certain inferred concepts, the object of which can never be given empirically.
The three relations of representations of which we can form either a concept or idea:
The relation to the subject.
The relation to the manifold of the object in the field of appearance.