Spinoza and Leibniz belong to another age. Modern philosophy began with a new emphasis upon self-consciousness. In his celebrated argument--"I think, hence I am" (_cogito ergo sum_)--Descartes established the independent and substantial reality of the thinking activity. The "I think" is recognized as in itself a fundamental being, known intuitively to the thinker himself. Now although Spinoza and Leibniz are finally determined by the same motives that obtain in the cases of Plato and Aristotle, they must reckon with this new distinction between the thinker and his object. The result in the case of Spinoza is the doctrine of "parallelism," in which mind is defined as an "infinite attribute" of substance, an aspect or phase coextensive with the whole of being. The result in the case of Leibniz is his doctrine of "representation" and "preëstablished harmony," whereby each monadic substance is in itself an active spiritual entity, and belongs to the universe through its knowledge of a specific stage of the development of the universe. But both Spinoza and Leibniz subordinate such conceptions as these to the fundamental identity that pervades the whole. With Spinoza the attributes belong to the same absolute substance, and with Leibniz the monads represent the one universe. And with both, finally, the perfection of knowledge, or the knowledge of God, is indistinguishable from its object, God himself. The epistemological subtleties peculiar to these philosophers are not stable doctrines, but render inevitable either a return to the simpler and bolder realism of the Greeks, or a passing over into the more radical and systematic doctrine of absolute idealism. [Sidenote: The Stoic and Spinozistic Ethics of Necessity.] § 168. We have met with two general motives, both of which are subordinated to the doctrine of an absolute being postulated and sought by philosophy. The one of these motives leads to the conception of the absolutely necessary and immutable substance, the other to the conception of a consummate perfection. There is an _interpretation of life_ appropriate to each of these conceptions. Both agree in regarding life seriously, in defining reason or philosophy as the highest human activity, and in emphasizing the identity of the individual's good with the good of the universe. But there are striking differences of tone and spirit.