All our knowledge begins with sense, proceeds thence to understanding, and ends with reason, beyond which nothing higher can be discovered in the human mind for elaborating the matter of intuition and subjecting it to the highest unity of thought. At this stage of our inquiry it is my duty to give an explanation of this, the highest faculty of cognition, and I confess I find myself here in some difficulty. Of reason, as of the understanding, there is a merely formal, that is, logical use, in which it makes abstraction of all content of cognition; but there is also a real use, inasmuch as it contains in itself the source of certain conceptions and principles, which it does not borrow either from the senses or the understanding. The former faculty has been long defined by logicians as the faculty of mediate conclusion in contradistinction to immediate conclusions (consequentiae immediatae); but the nature of the latter, which itself generates conceptions, is not to be understood from this definition. Now as a division of reason into a logical and a transcendental faculty presents itself here, it becomes necessary to seek for a higher conception of this source of cognition which shall comprehend both conceptions. In this we may expect, according to the analogy of the conceptions of the understanding, that the logical conception will give us the key to the transcendental, and that the table of the functions of the former will present us with the clue to the conceptions of reason.
In the former part of our transcendental logic, we defined the understanding to be the faculty of rules; reason may be distinguished from understanding as the faculty of principles.
The term principle is ambiguous, and commonly signifies merely a cognition that may be employed as a principle, although it is not in itself, and as regards its proper origin, entitled to the distinction. Every general proposition, even if derived from experience by the process of induction, may serve as the major in a syllogism; but it is not for that reason a principle. Mathematical axioms (for example, there can be only one straight line between two points) are general a priori cognitions, and are therefore rightly denominated principles, relatively to the cases which can be subsumed under them. But I cannot for this reason say that I cognize this property of a straight line from principles- I cognize it only in pure intuition.
Cognition from principles, then, is that cognition in which I cognize the particular in the general by means of conceptions. Thus every syllogism is a form of the deduction of a cognition from a principle. For the major always gives a conception, through which everything that is subsumed under the condition thereof is cognized according to a principle. Now as every general cognition may serve as the major in a syllogism, and the understanding presents us with such general a priori propositions, they may be termed principles, in respect of their possible use.
But if we consider these principles of the pure understanding in relation to their origin, we shall find them to be anything rather than cognitions from conceptions. For they would not even be possible a priori, if we could not rely on the assistance of pure intuition (in mathematics), or on that of the conditions of a possible experience. That everything that happens has a cause, cannot be concluded from the general conception of that which happens; on the contrary the principle of causality instructs us as to the mode of obtaining from that which happens a determinate empirical conception.
Synthetical cognitions from conceptions the understanding cannot supply, and they alone are entitled to be called principles. At the same time, all general propositions may be termed comparative principles.
It has been a long-cherished wish- that (who knows how late), may one day, be happily accomplished- that the principles of the endless variety of civil laws should be investigated and exposed; for in this way alone can we find the secret of simplifying legislation. But in this case, laws are nothing more than limitations of our freedom upon conditions under which it subsists in perfect harmony with itself; they consequently have for their object that which is completely our own work, and of which we ourselves may be the cause by means of these conceptions. But how objects as things in themselves- how the nature of things is subordinated to principles and is to be determined. according to conceptions, is a question which it seems well nigh impossible to answer. Be this, however, as it may- for on this point our investigation is yet to be made- it is at least manifest from what we have said that cognition from principles is something very different from cognition by means of the understanding, which may indeed precede other cognitions in the form of a principle, but in itself- in so far as it is synthetical- is neither based upon mere thought, nor contains a general proposition drawn from conceptions alone shall comprehend
The understanding may be a faculty for the production of unity of phenomena by virtue of rules; the reason is a faculty for the production of unity of rules (of the understanding) under principles. Reason, therefore, never applies directly to experience, or to any sensuous object; its object is, on the contrary, the understanding, to the manifold cognition of which it gives a unity a priori by means of conceptions- a unity which may be called rational unity, and which is of a nature very different from that of the unity produced by the understanding.
The above is the general conception of the faculty of reason, in so far as it has been possible to make it comprehensible in the absence of examples. These will be given in the sequel.