is real as independent of us and therefore as something which we may discover but can in no sense make, we tend to think of the object of thought as only an idea. On the other hand, what is ordinarily called perception, though it involves the activity of thinking, also involves an element in respect of which we are passive. This is the fact pointed to by Kant's phrase 'objects are _given_ in perception'. In virtue of this passive element we are inclined to think that in perception we simply stand before the reality in a passive attitude. The reality perceived is thought to be, so to say, there, existing independently of us; relation to the subject is unnoticed because of our apparently wholly passive attitude. At times, and especially when he is thinking of the understanding as a faculty of spontaneity, Kant seems to have been under the influence of this second tendency. [18] i. e. as not having a place in the reality which, as we think, exists independently of the mind. The preceding summary of the problem of the _Critique_ represents the account given in the two Prefaces and the Introduction. According to this account, the problem arises from the unquestioned existence of _a priori_ knowledge in mathematics and physics and the problematic existence of such knowledge in metaphysics, and Kant's aim is to determine the range within which _a priori_ knowledge is possible. Thus the problem is introduced as relating to _a priori_ knowledge as such, no distinction being drawn between its character in different cases. Nevertheless the actual discussion of the problem in the body of the _Critique_ implies a fundamental distinction between the nature of _a priori_ knowledge in mathematics and its nature in physics, and in order that a complete view of the problem may be given, this distinction must be stated.