We termed dialectic in general a logic of appearance. This does not signify a doctrine of probability; for probability is truth, only cognized upon insufficient grounds, and though the information it gives us is imperfect, it is not therefore deceitful. Hence it must not be separated from the analytical part of logic. Still less must phenomenon and appearance be held to be identical. For truth or illusory appearance does not reside in the object, in so far as it is intuited, but in the judgement upon the object, in so far as it is thought. It is, therefore, quite correct to say that the senses do not err, not because they always judge correctly, but because they do not judge at all. Hence truth and error, consequently also, illusory appearance as the cause of error, are only to be found in a judgement, that is, in the relation of an object to our understanding. In a cognition which completely harmonizes with the laws of the understanding, no error can exist. In a representation of the senses- as not containing any judgement- there is also no error. But no power of nature can of itself deviate from its own laws. Hence neither the understanding per se (without the influence of another cause), nor the senses per se, would fall into error; the former could not, because, if it acts only according to its own laws, the effect (the judgement) must necessarily accord with these laws. But in accordance with the laws of the understanding consists the formal element in all truth. In the senses there is no judgement- neither a true nor a false one. But, as we have no source of cognition besides these two, it follows that error is caused solely by the unobserved influence of the sensibility upon the understanding. And thus it happens that the subjective grounds of a judgement and are confounded with the objective, and cause them to deviate from their proper determination,* just as a body in motion would always of itself proceed in a straight line, but if another impetus gives to it a different direction, it will then start off into a curvilinear line of motion. To distinguish the peculiar action of the understanding from the power which mingles with it, it is necessary to consider an erroneous judgement as the diagonal between two forces, that determine the judgement in two different directions, which, as it were, form an angle, and to resolve this composite operation into the simple ones of the understanding and the sensibility. In pure a priori judgements this must be done by means of transcendental reflection, whereby, as has been already shown, each representation has its place appointed in the corresponding faculty of cognition, and consequently the influence of the one faculty upon the other is made apparent.
*Sensibility, subjected to the understanding, as the object upon which the understanding employs its functions, is the source of real cognitions. But, in so far as it exercises an influence upon the action of the understanding and determines it to judgement, sensibility is itself the cause of error.
It is not at present our business to treat of empirical illusory appearance (for example, optical illusion), which occurs in the empirical application of otherwise correct rules of the understanding, and in which the judgement is misled by the influence of imagination. Our purpose is to speak of transcendental illusory appearance, which influences principles- that are not even applied to experience, for in this case we should possess a sure test of their correctness- but which leads us, in disregard of all the warnings of criticism, completely beyond the empirical employment of the categories and deludes us with the chimera of an extension of the sphere of the pure understanding. We shall term those principles the application of which is confined entirely within the limits of possible experience, immanent; those, on the other hand, which transgress these limits, we shall call transcendent principles. But by these latter I do not understand principles of the transcendental use or misuse of the categories, which is in reality a mere fault of the judgement when not under due restraint from criticism, and therefore not paying sufficient attention to the limits of the sphere in which the pure understanding is allowed to exercise its functions; but real principles which exhort us to break down all those barriers, and to lay claim to a perfectly new field of cognition, which recognizes no line of demarcation. Thus transcendental and transcendent are not identical terms. The principles of the pure understanding, which we have already propounded, ought to be of empirical and not of transcendental use, that is, they are not applicable to any object beyond the sphere of experience. A principle which removes these limits, nay, which authorizes us to overstep them, is called transcendent. If our criticism can succeed in exposing the illusion in these pretended principles, those which are limited in their employment to the sphere of experience may be called, in opposition to the others, immanent principles of the pure understanding.
Logical illusion, which consists merely in the imitation of the form of reason (the illusion in sophistical syllogisms), arises entirely from a want of due attention to logical rules. So soon as the attention is awakened to the case before us, this illusion totally disappears. Transcendental illusion, on the contrary, does not cease to exist, even after it has been exposed, and its nothingness clearly perceived by means of transcendental criticism. Take, for example, the illusion in the proposition: "The world must have a beginning in time." The cause of this is as follows. In our reason, subjectively considered as a faculty of human cognition, there exist fundamental rules and maxims of its exercise, which have completely the appearance of objective principles. Now from this cause it happens that the subjective necessity of a certain connection of our conceptions, is regarded as an objective necessity of the determination of things in themselves. This illusion it is impossible to avoid, just as we cannot avoid perceiving that the sea appears to be higher at a distance than it is near the shore, because we see the former by means of higher rays than the latter, or, which is a still stronger case, as even the astronomer cannot prevent himself from seeing the moon larger at its rising than some time afterwards, although he is not deceived by this illusion.
Transcendental dialectic will therefore content itself with exposing the illusory appearance in transcendental judgements, and guarding us against it; but to make it, as in the case of logical illusion, entirely disappear and cease to be illusion is utterly beyond its power. For we have here to do with a natural and unavoidable illusion, which rests upon subjective principles and imposes these upon us as objective, while logical dialectic, in the detection of sophisms, has to do merely with an error in the logical consequence of the propositions, or with an artificially constructed illusion, in imitation of the natural error. There is, therefore, a natural and unavoidable dialectic of pure reason- not that in which the bungler, from want of the requisite knowledge, involves himself, nor that which the sophist devises for the purpose of misleading, but that which is an inseparable adjunct of human reason, and which, even after its illusions have been exposed, does not cease to deceive, and continually to lead reason into momentary errors, which it becomes necessary continually to remove.