Reflection (reflexio) is not occupied about objects themselves, for the purpose of directly obtaining conceptions of them, but is that state of the mind in which we set ourselves to discover the subjective conditions under which we obtain conceptions. It is the consciousness of the relation of given representations to the different sources or faculties of cognition, by which alone their relation to each other can be rightly determined. The first question which occurs in considering our representations is to what faculty of cognition do they belong? To the understanding or to the senses? Many judgements are admitted to be true from mere habit or inclination; but, because reflection neither precedes nor follows, it is held to be a judgement that has its origin in the understanding. All judgements do not require examination, that is, investigation into the grounds of their truth. For, when they are immediately certain (for example: "Between two points there can be only one straight line"), no better or less mediate test of their truth can be found than that which they themselves contain and express. But all judgement, nay, all comparisons require reflection, that is, a distinction of the faculty of cognition to which the given conceptions belong. The act whereby I compare my representations with the faculty of cognition which originates them, and whereby I distinguish whether they are compared with each other as belonging to the pure understanding or to sensuous intuition, I term transcendental reflection. Now, the relations in which conceptions can stand to each other are those of identity and difference, agreement and opposition, of the internal and external, finally, of the determinable and the determining (matter and form). The proper determination of these relations rests on the question, to what faculty of cognition they subjectively belong, whether to sensibility or understanding? For, on the manner in which we solve this question depends the manner in which we must cogitate these relations.
Before constructing any objective judgement, we compare the conceptions that are to be placed in the judgement, and observe whether there exists identity (of many representations in one conception), if a general judgement is to be constructed, or difference, if a particular; whether there is agreement when affirmative; and opposition when negative judgements are to be constructed, and so on. For this reason we ought to call these conceptions, conceptions of comparison (conceptus comparationis). But as, when the question is not as to the logical form, but as to the content of conceptions, that is to say, whether the things themselves are identical or different, in agreement or opposition, and so on, the things can have a twofold relation to our faculty of cognition, to wit, a relation either to sensibility or to the understanding, and as on this relation depends their relation to each other, transcendental reflection, that is, the relation of given representations to one or the other faculty of cognition, can alone determine this latter relation. Thus we shall not be able to discover whether the things are identical or different, in agreement or opposition, etc., from the mere conception of the things by means of comparison (comparatio), but only by distinguishing the mode of cognition to which they belong, in other words, by means of transcendental reflection. We may, therefore, with justice say, that logical reflection is mere comparison, for in it no account is taken of the faculty of cognition to which the given conceptions belong, and they are consequently, as far as regards their origin, to be treated as homogeneous; while transcendental reflection (which applies to the objects themselves) contains the ground of the possibility of objective comparison of representations with each other, and is therefore very different from the former, because the faculties of cognition to which they belong are not even the same. Transcendental reflection is a duty which no one can neglect who wishes to establish an a priori judgement upon things. We shall now proceed to fulfil this duty, and thereby throw not a little light on the question as to the determination of the proper business of the understanding.
1. Identity and Difference. When an object is presented to us several times, but always with the same internal determinations (qualitas et quantitas), it, if an object of pure understanding, is always the same, not several things, but only one thing (numerica identitas); but if a phenomenon, we do not concern ourselves with comparing the conception of the thing with the conception of some other, but, although they may be in this respect perfectly the same, the difference of place at the same time is a sufficient ground for asserting the numerical difference of these objects (of sense). Thus, in the case of two drops of water, we may make complete abstraction of all internal difference (quality and quantity), and, the fact that they are intuited at the same time in different places, is sufficient to justify us in holding them to be numerically different. Leibnitz regarded phenomena as things in themselves, consequently as intelligibilia, that is, objects of pure understanding (although, on account of the confused nature of their representations, he gave them the name of phenomena), and in this case his principle of the indiscernible (principium identatis indiscernibilium) is not to be impugned. But, as phenomena are objects of sensibility, and, as the understanding, in respect of them, must be employed empirically and not purely or transcendentally, plurality and numerical difference are given by space itself as the condition of external phenomena. For one part of space, although it may be perfectly similar and equal to another part, is still without it, and for this reason alone is different from the latter, which is added to it in order to make up a greater space. It follows that this must hold good of all things that are in the different parts of space at the same time, however similar and equal one may be to another.
2. Agreement and Opposition. When reality is represented by the pure understanding (realitas noumenon), opposition between realities is incogitable- such a relation, that is, that when these realities are connected in one subject, they annihilate the effects of each other and may be represented in the formula 3 - 3 = 0. On the other hand, the real in a phenomenon (realitas phaenomenon) may very well be in mutual opposition, and, when united in the same subject, the one may completely or in part annihilate the effect or consequence of the other; as in the case of two moving forces in the same straight line drawing or impelling a point in opposite directions, or in the case of a pleasure counterbalancing a certain amount of pain.
3. The Internal and External. In an object of the pure understanding, only that is internal which has no relation (as regards its existence) to anything different from itself. On the other hand, the internal determinations of a substantia phaenomenon in space are nothing but relations, and it is itself nothing more than a complex of mere relations. Substance in space we are cognizant of only through forces operative in it, either drawing others towards itself (attraction), or preventing others from forcing into itself (repulsion and impenetrability). We know no other properties that make up the conception of substance phenomenal in space, and which we term matter. On the other hand, as an object of the pure understanding, every substance must have internal determination and forces. But what other internal attributes of such an object can I think than those which my internal sense presents to me? That, to wit, which in either itself thought, or something analogous to it. Hence Leibnitz, who looked upon things as noumena, after denying them everything like external relation, and therefore also composition or combination, declared that all substances, even the component parts of matter, were simple substances with powers of representation, in one word, monads.
4. Matter and Form. These two conceptions lie at the foundation of all other reflection, so inseparably are they connected with every mode of exercising the understanding. The former denotes the determinable in general, the second its determination, both in a transcendental sense, abstraction being made of every difference in that which is given, and of the mode in which it is determined. Logicians formerly termed the universal, matter, the specific difference of this or that part of the universal, form. In a judgement one may call the given conceptions logical matter (for the judgement), the relation of these to each other (by means of the copula), the form of the judgement. In an object, the composite parts thereof (essentialia) are the matter; the mode in which they are connected in the object, the form. In respect to things in general, unlimited reality was regarded as the matter of all possibility, the limitation thereof (negation) as the form, by which one thing is distinguished from another according to transcendental conceptions. The understanding demands that something be given (at least in the conception), in order to be able to determine it in a certain manner. Hence, in a conception of the pure understanding, the matter precedes the form, and for this reason Leibnitz first assumed the existence of things (monads) and of an internal power of representation in them, in order to found upon this their external relation and the community their state (that is, of their representations). Hence, with him, space and time were possible- the former through the relation of substances, the latter through the connection of their determinations with each other, as causes and effects. And so would it really be, if the pure understanding were capable of an immediate application to objects, and if space and time were determinations of things in themselves. But being merely sensuous intuitions, in which we determine all objects solely as phenomena, the form of intuition (as a subjective property of sensibility) must antecede all matter (sensations), consequently space and time must antecede all phenomena and all data of experience, and rather make experience itself possible. But the intellectual philosopher could not endure that the form should precede the things themselves and determine their possibility; an objection perfectly correct, if we assume that we intuite things as they are, although with confused representation. But as sensuous intuition is a peculiar subjective condition, which is a priori at the foundation of all perception, and the form of which is primitive, the form must be given per se, and so far from matter (or the things themselves which appear) lying at the foundation of experience (as we must conclude, if we judge by mere conceptions), the very possibility of itself presupposes, on the contrary, a given formal intuition (space and time).