The beginning of Kant's account of schematism may be summarized thus: 'Whenever we subsume an individual object of a certain kind, e. g. a plate, under a conception, e. g. a circle, the object and the conception must be homogeneous, that is to say, the individual must possess the characteristic which constitutes the conception, or, in other words, must be an instance of it. Pure conceptions, however, and empirical perceptions, i. e. objects of empirical perception, are quite heterogeneous. We do not, for instance, perceive cases of cause and effect. Hence the problem arises, 'How is it possible to subsume objects of empirical perception under pure conceptions?' The possibility of this subsumption presupposes a _tertium quid_, which is homogeneous both with the object of empirical perception and with the conception, and so makes the subsumption mediately possible. This _tertium quid_ must be, on the one side, intellectual and, on the other side, sensuous. It is to be found in a 'transcendental determination of time', i. e. a conception involving time and involved in experience. For in the first place this is on the one side intellectual and on the other sensuous, and in the second place it is so far homogeneous with the category which constitutes its unity that it is universal and rests on an _a priori_ rule, and so far homogeneous with the phenomenon that all phenomena are in time[5]. Such transcendental determinations of time are the schemata of the pure conceptions of the understanding.' Kant continues as follows: [5] It may be noted that the argument here really fails. For though phenomena as involving temporal relations, might possibly be said to be instances of a transcendental determination of time, the fact that the latter agrees with the corresponding category by being universal and _a priori_ does not constitute it homogeneous with the category, in the sense required for subsumption, viz. that it is an instance of or a species of the category. "The schema is in itself always a mere product of the imagination. But since the synthesis of the imagination has for its aim no single perception, but merely unity in the determination of the sensibility, the schema should be distinguished from the image. Thus, if I place five points one after another, . . . . . this is an image of the number five. On the other hand, if I only just think a number in general--no matter what it may be, five or a hundred--this thinking is rather the representation of a method of representing in an image a group (e. g. a thousand), in conformity with a certain conception, than the image itself, an image which, in the instance given, I should find difficulty in surveying and comparing with the conception. Now this representation of a general procedure of the imagination to supply its image to a conception, I call the schema of this conception."