_I posit in the Ego a divisible non Ego over against the divisible Ego._ No philosophy can go beyond this cognition, and every fundamental philosophy should go back to this. Just so far as it does this, it becomes science (_Wissenschaftslehre_). Every thing which can appear in a system of knowledge, as well as a farther division of the Theory of Science itself, must be derived from this. The proposition that the Ego and the non-Ego reciprocally limit each other, may be divided into the following two: (1) the Ego posits itself as limited through the non-Ego (_i. e._ the Ego is in a cognitive (or passive) relation); (2) the Ego posits the non-Ego as limited through the Ego (_i. e._ the Ego is in an active relation). The former proposition is the basis of the theoretical, and the latter of the practical part of the Theory of Science. The latter part cannot, at the outset, be brought upon the stage; for the non-Ego, which should be limited by the acting Ego, does not at the outset exist, and we must wait and see whether it will find, in the theoretical part, a reality.
_The groundwork of theoretical knowledge_ advances through an uninterrupted series of antitheses and syntheses. The fundamental synthesis of the theoretical Theory of Science is the proposition: _the Ego posits itself as determined_ (limited) _by the non-Ego_. If we analyze this sentence, we find in it two subordinate sentences which are reciprocally opposite. (1) The non-Ego as active determines the Ego, which thus far is passive; but since all activity must start from the Ego, so (2) the Ego determines itself through an absolute activity. Herein is a contradiction, that the Ego should be at the same time active and passive. Since this contradiction would destroy the above proposition, and also suppress the unity of consciousness, we are forced to seek some point, some new synthesis, in which these given antitheses may be united. This synthesis is attained when we find that the conceptions of action and passion, which are contained under the categories of reality and negation, find their compensation and due adjustment in the conception of divisibility. The propositions: “the Ego determines,” and “the Ego is determined,” are reconciled in the proposition: “the Ego determines itself in part, and is determined in part.” Both, however, should be considered as one and the same. Hence more accurately: as many parts of reality as the Ego posits in itself, so many parts of negation does it posit in the non-Ego; and as many parts of reality as the Ego posits in the non-Ego, so many parts of negation does it posit in itself. This determination is _reciprocal determination_, or _reciprocal action_. Thus Fichte deduces the last of the three categories under Kant’s general category of relation. In a similar way (viz., by finding a synthesis for apparent contradictions), he deduces the two other categories of this class, viz., that of cause, and that of substance. The process is thus: So far as the Ego is determined, and therefore passive, has the non-Ego reality. The category of reciprocal determination, to which we may ascribe indifferently either of the two sides, reality or negation, may, more strictly taken, imply that the Ego is passive, and the non-Ego active. The notion which expresses this relation is that of _causality_. That, to which activity is ascribed, is called _cause_ (primal reality), and that to which passiveness is ascribed, is called _effect_; both, conceived in connection, may be termed a _working_. On the other side, the Ego determines itself. Herein is a contradiction; (1) the Ego determines itself; it is therefore that which determines, and is thus active; (2) it determines itself; it is therefore that which becomes determined, and is thus passive. Thus in one respect and in one action both reality and negation are ascribed to it. To resolve this contradiction, we must find a mode of action which is activity and passiveness in one; the Ego must determine its passiveness through activity, and its activity through passiveness. This solution is attained by aid of the conception of quantity. In the Ego all reality is first of all posited as absolute quantum, as absolute totality, and thus far the Ego may be compared to a greatest circle which contains all the rest. A definite quantum of activity, or a limited sphere within this greatest circle of activity, is indeed a _reality_; but when compared with the totality of activity, is it also a _negation_ of the totality or passiveness. Here we have found the mediation sought for; it lies in the notion of _substance_.