The conflict centers, first, about the question concerning the origin of human knowledge and the sphere of its validity.  Rationalism is justified when it asserts that some ideas do not come from the senses.  If knowledge is to be possible, some concepts cannot originate in perception, those, namely, by which knowledge is constituted, for if they should, it would lack universality and necessity.  The sole organ of universally valid knowledge is reason.  Empiricism, on the other hand, is justified when it asserts that the experiential alone is knowable.  Whatever is to be knowable must be given as a real in sensuous intuition.  The only organ of reality is sensibility.  Rationalism judges correctly concerning the origin of the most important classes of ideas; empiricism concerning the sphere of their validity.  The two may be thus combined:  some concepts (those which produce knowledge) take their origin in reason or are a priori, but they are valid for objects of experience alone.  The conflict concerns, secondly, the use of the deductive (syllogistic) or the inductive method.  Empiricism, through its founder Bacon, had recommended induction in place of the barren syllogistic method, as the only method which would lead to new discoveries.  It demands, above all things, the extension of knowledge.  Rationalism, on the contrary, held fast to the deductive method, because the syllogism alone, in its view, furnishes knowledge valid for all rational beings.  It demands, first of all, universality and necessity in knowledge.  Induction has the advantage of increasing knowledge, but it leads only to empirical and comparative, not to strict universality.  The syllogism has the advantage of yielding universal and necessary truth, but it can only explicate and establish knowledge, not increase it.  May it not be possible so to do justice to the demands of both that the advantages which they seek shall be combined, and the disadvantages which have been feared, avoided?  Are there not cognitions which increase our knowledge (are synthetic) without being empirical, which are universally and necessarily valid (a priori) without being analytic?  From these considerations arises the main question of the Critique of Pure Reason:  How are synthetic judgments a priori possible?

The philosophy of experience had overestimated sense and underestimated the understanding, when it found the source of all knowledge in the faculty of perception and degraded the faculty of thought to an almost wholly inactive recipient of messages coming to it from without.  From the standpoint of empiricism concepts (Ideas) deserve confidence only in so far as they can legitimate themselves by their origin in sensations (impressions).  It overlooks the active character of all knowing.  Among the rationalists, on the other hand, we find an underestimation of the senses and an overestimation of the understanding.  They believe that sense