From the fact that we are able spontaneously to assign and determine the laws of relations in space without having recourse to experience, Plato concludes (_Meno_, p. 353, Bip.) that all learning is mere recollection. Kant, on the other hand, concludes that space is subjectively conditioned, and merely a form of the faculty of knowledge. How far, in this regard, does Kant stand above Plato! _Cogito, ergo sum_, is an analytical judgment. Indeed Parmenides held it to be an identical judgment: “το γαρ αυτο νοειν εστι τε και ειναι” (_nam intelligere et esse idem est_, _Clem. Alex. Strom._, vi. 2, § 23). As such, however, or indeed even as an analytical judgment, it cannot contain any special wisdom; nor yet if, to go still deeper, we seek to deduce it as a conclusion from the major premise, _non-entis nulla sunt prædicata_. But with this proposition what Descartes really wished to express was the great truth that immediate certainty belongs only to self-consciousness, to what is subjective. To what is objective, on the other hand, thus to everything else, only indirect certainty belongs; for it is arrived at through self-consciousness; and being thus merely at second hand, it is to be regarded as problematical. Upon this depends the value of this celebrated proposition. As its opposite we may set up, in the sense of the Kantian philosophy, _cogito, ergo est_, that is, exactly as I think certain relations in things (the mathematical), they must always occur in all possible experience;—this was an important, profound, and a late _apperçu_, which appeared in the form of the problem as to the _possibility of synthetic judgments a priori_, and has actually opened up the way to a deeper knowledge. This problem is the watchword of the Kantian philosophy, as the former proposition is that of the Cartesian, and shows εξ οἱων εισ οἱα. Kant very fitly places his investigations concerning time and space at the head of all the rest. For to the speculative mind these questions present themselves before all others: what is time?—what is this that consists of mere movement, without anything that moves it?—and what is space? this omnipresent nothing, out of which nothing that exists can escape without ceasing to be anything at all? That time and space depend on the subject, are the mode in which the process of objective apperception is brought about in the brain, has already a sufficient proof in the absolute impossibility of thinking away time and space, while we can very easily think away everything that is presented in them. The hand can leave go of everything except itself. However, I wish here to illustrate by a few examples and deductions the more exact proofs of this truth which are given by Kant, not for the purpose of refuting stupid objections, but for the use of those who may have to expound Kant’s doctrine in future.