Page:A Brief History of Modern Philosophy.djvu/89

 effort. Leibnitz likewise discovers a tendency (appetite-tendance) in the soul, to pass from the single “perceptions” to new perceptions. This is an element which presupposes other distinctions than obscurity and clearness. Both Spinoza and Leibnitz contain suggestions of a profounder theory of will, which is suppressed by their intellectualistic tendency.

e. Although Leibnitz, in opposition to Locke, maintains the involuntary and unconscious foundation of knowledge, and objected to the idea of a tabula rasa, he is still in agreement with Locke’s criticism of “innate ideas” in requiring a proof for all truths, even the “innate,” that are not identical propositions. To prove a proposition means to trace it back to an identical proposition. According to him logic culminates in the principle of identity whilst the Aristotelians and Scholastics base their theory on the principle of contradiction. He had sketched an outline of logic in which each judgment is stated in the form of an identical proposition. But this sketch was unknown until 1840 (in J. E. Erdmann’s Opera philosophica Leibnitii), and the logical investigations of Boole and Jevons, which reveal a similar tendency, were the first to direct attention to them.

Just as the principle of identity is the criterion of truth in the realm of pure thought, so is the principle of sufficient reason in the realm of experience. Leibnitz however, even as Spinoza, never made a clear distinction between ground and cause (ratio and causa). He regarded this principle not only as a principle of scientific investigation, but as a universal law.—The difference between truths of experience (“contingent” truths) and truths of pure thought (“necessary” truths) is only a matter of degree: the former can be reduced to