This becomes clear if we consider the role of time and space in the Kantian system: they are foundational in the psychology of perception, from them much else is deduced, and upon them much else is based. Time and space hold a prior position in Kantian metaphysics and epistemology, not only in the sense that they are located toward the front of the book (The Critique of Pure Reason), but also in the sense that they are the necessary condition for experience, perception, representation, and intuition. Time and space are not “real” in the sense of being noumenal, but rather are the structure or framework of the mind - or at least a part of such structure or framework - the machinery by which we process our sense-data into perceptions.
Bearing in mind, then, the centrality of space and time to the Kantian system, we see that Fichte writes, concerning those who consider ethical systems from a roughly Kantian point of view, that
I would like very much to know what those who assume a familiar air of superiority whenever they encounter any mention of “intelligible intuition” imagine our consciousness of the ethical law to be like, or how they are able to construct for themselves concepts such as “right,” “virtue,” and the like - concepts that they certainly to possess. According to them, there are but two a priori intuitions: time and space. They undoubtedly construct the [ethical] concepts in question within time, which is the form of inner sense. But they undoubtedly do not consider these concepts to be identical to time itself; they are only a certain way of filling time. With what then do they fill time in this case, and what underlies their construction of these concepts? The only a priori intuition that remains for them is space. Consequently, their “right” would have to turn out to be, let us say, square, while their “virtue” would perhaps have to be circular, in the same way that all of the concepts they construct on the basis of sensory intuition - the concept of a tree, for example, or of an animal - are nothing but certain ways of limiting space. But they do not really conceive of right or virtue in this manner. What then is it that underlies their construction of these concepts? If they observe correctly, they will discover that what underlies these concepts is acting as such, i.e., freedom. Both concepts, the concept of right as well as that of virtue, are for them determinate limitations of acting as such, just as all sensory concepts are for them determinate limitations of space. But how have they arrived at this foundation that underlies their construction of these concepts? One would hope that they have not inferred acting from the dead inertia of matter, nor freedom from the mechanism of nature. They must have obtained this by means of immediate intuition; consequently, in addition to the two intuition they recognize, there must also be a third.
Fichte points out that the quasi-Kantian ethicists cannot find an adequate foundation for moral thought in the two intuitions of time and space. (He seems careful not to attack Kant himself directly, either because he genuinely thinks that Kant is free from the errors of the later Kantians, or because Fichte’s respect for Kant is too great, or because a direct attack on Kant would not have been tolerated by Fichte’s contemporaries.) If time and space are the starting point for all mental processes, then there could be no moral thought: if there is moral thought, then there must be a third foundational intuition alongside the other two.
So Fichte builds his system on a three-legged stool, a philosophical tripod, and sees this as an advantage over the two-legged foundation which is supposed to support the Kantian system. Kant began his epistemology with the notion that space and time would be the framework, or would generate the framework, which the mind would use to perceive and understand. Fichte adds a third atom to this framework: an ethical intuition. This intuition will deal in some fashion with “action” and “freedom”.
Thus constructed, Fichte’s ethics will find their roots in the very foundations of his system. If the three feet of his system are time, space, and ethical intuition, then metaphysics and ethics will play the most central role in the system, relegating other types of philosophy to secondary roles, and Copleston will be correct in observing that Fichte has a “tendency ... to reduce religion to morals,” inasmuch as ethics are more foundational to the system than God is.
Offering a slightly different explanation of his addition to the Kantian system, Fichte reviews the attempts of Kantians to explain the self in the same manner in which they explain noumenal or phenomenal objects. Fichte objects that, if we treat the self the same way we treat a rock or a tree, we will miss important aspects of the self which make it a subject instead of merely an object.
If such an existence were ascribed to the self, it would cease to be a self; it would become a thing, and its concept would be abolished. Later, to be sure - not in time, but in order of dependence of thought - even the self, though still remaining, as it must, a self in our sense of the word, will also be credited with existence of this sort; in part with extension and subsistence in space, and in this respect it becomes a determinate body; in part with identity and duration in time, and in this respect it becomes a soul. But it is the business of philosophy to demonstrate and explain genetically how the self comes to thing of itself in this fashion; and so all this belongs, not to what we have to assume, but to what we have to derive. - It comes, then, to this: the self is originally a doing, merely; even if we think of it only as active, we already have an empirical concept of it, and so one that has first to be derived.
We have two arguments here: first, that we cannot create or justify the foundation of ethical concepts without adding a third intuition to the first two, i.e., adding “action” to time and space; second, we cannot explain or sufficiently express the nature of the self, i.e as active agent and knowing subject, without this third leg to the stool.
Fichte attempts - whether he succeeds or not, the reader may judge - to show that an accurate concept of self is impossible without his system:
The principle of life and consciousness, the ground of its possibility - is admittedly contained in the self; but this gives rise to no genuine life, no empirical existence in time; and any other kind, for us, is absolutely unthinkable. If such a genuine life is to be possible, we need for the purpose another and special sort of check to the self on the part of the not-self.
Fichte sees himself less as contradicting Kant, and more as extracting from Kant’s system that which is already hidden or latent within it. Thus he sees that Kant’s critical philosophy begins with the exploration of the mechanisms of the mind - e.g., how time and space are parts of the mind, or generated by the mind, and used by the mind to process experiences and sense-data into perceptions and representations. The mind is, then, essentially active; from this, Fichte proceeds to identify “action” as a fundamental or foundational intuition.
