The early success of logic must be attributed exclusively to the narrowness of its field, in which abstraction may, or rather must, be made of all the objects of cognition with their characteristic distinctions, and in which the understanding has only to deal with itself and with its own forms. It is, obviously, a much more difficult task for reason to strike into the sure path of science, where it has to deal not simply with itself, but with objects external to itself. Hence, logic is properly only a _propædeutic_—forms, as it were, the vestibule of the sciences; and while it is necessary to enable us to form a correct judgement with regard to the various branches of knowledge, still the acquisition of real, substantive knowledge is to be sought only in the sciences properly so called, that is, in the objective sciences. Now these sciences, if they can be termed rational at all, must contain elements of _à priori_ cognition, and this cognition may stand in a twofold relation to its object. Either it may have to _determine_ the conception of the object—which must be supplied extraneously, or it may have to _establish its reality_. The former is _theoretical_, the latter _practical_, rational cognition. In both, the _pure_ or _à priori_ element must be treated first, and must be carefully distinguished from that which is supplied from other sources. Any other method can only lead to irremediable confusion. _Mathematics_ and _physics_ are the two theoretical sciences which have to determine their objects _à priori_. The former is purely _à priori_, the latter is partially so, but is also dependent on other sources of cognition.