The peculiar position in which Kant had left the theory of cognition was assailed from many different sides and by many writers, specially by Schultze (Aenesidemus) and Maimon.
The psychological theory of cognition takes for granted the dualism of the mind that knows and the object known; it takes no account of the metaphysical problem as to the possibility of a relation between the ego and the non-ego, but assumes that such a relation does exist.
Both cognition and volition are functions of thought as well as forms of moral action.
In cognition being is the object and in volition it is the purpose of thought: in the first case we receive (in our fashion) the object of thought into ourselves; in the latter we plant it out into the world.
The relation in which they stand to the categories or pure notions is ambiguous; and, when Kant has to consider the fashion in which category and data of sense are to be brought together, he merely places side by side as a priori elements the pure connective notions and the pure forms of perception, and finds it, apparently, only a matter of contingent convenience that they should harmonize with one another and so render cognition possible.
That the ego to which different parts of experience are presented must be the same ego, if there is to be cognition at all, is analytically evident; but the peculiarity is that the ego must be conscious of its own unity and identity, and this unity of self-consciousness is only possible in relation to difference not contained in the ego but given to it.
The difficulty of reconciling the two views is that which gives rise to much of the obscurity in Locke's treatment of the theory of knowledge; in Hume the effort to identify them, and to explain the synthesis which is essential to cognition as merely the accidental result of external relations among the elements of conscious experience, appears with the utmost clearness, and gives the keynote of all his philosophical work.
For a complete treatment of this portion of the theory of knowledge, there require to be taken into consideration at least the following points: (a) the exact nature and significance of the space and time relations in our experience, (b) the mode in which the primary data, facts or principles, of mathematical cognition are obtained, (c) the nature, extent and certainty of such data, in themselves and with reference to the concrete material of experience, (d) the principle of inference from the data, however obtained.
Now if this mode of treatment be accepted as the only possible method, and its results assumed to be conclusive as regards the problem of knowledge, the fundamental peculiarity of cognition is overlooked.
It was left for Hume to approach the theory of knowledge with full consciousness from the psychological point of view, and to work out the final consequences of that view so far as cognition is concerned.