Gottfried Wilhelm Leibniz

Gottfried Wilhelm Leibniz   (July 1, 1646 – November 14, 1716) was a German philosopher and mathematician. In regards to the former role, Leibiniz was one of the three greatest advocates of rationalism in the 17th century, along with Rene Descartes and Baruch Spinozoa. His philosophy is mostly noted for its theistic optimism: i.e., for his conclusion that the universe we find ourselves in was the best of all worlds that God could have created. In regards to the latter role, Leibniz  developed the calculus of infinitesmals contemporaneous yet indepedently of Sir Isaac Newton, and his clear mathematical notion is generally used in calculus textbooks even to this today (in lieu of Newton's rather more complicated, deliberately obscure form of notation).

Metaphysics
From Leibniz's perspective, the fundamental questions of metaphysics were essentially reducible to ontological questions, predicated on one's answer to such questions as "What is?", "What are the most basic components of reality?", and "What grounds what else?"

His answer to this quesion throughout life was everything is composed or reducible to simple substances; everything could be grounded in simple substances. It was his belief that each substance had a complete concept individual to itself and that each was essentially an active unity capable of perception and appetition.

In his Discourse on Metaphysics, Leibniz claims that the Aristotlean understanding of substance as being that which is the subject of predication and which cannot be predicated of something else was insufficient to fully describing the nature of substance. Appealing to the Principle of Contradiction and the Predicate-in-Notion Principle, he then made a case that the concept of the predicate is contained in the concept of the subject in every true predication. From this, he concludes that the "nature of an individual substance or of a complete being is to have a notion so complete that it is sufficient to contain and to allow us to deduce from it all the predicates of the subject to which this notion is attributed.” Putting it differently, y is a substance if and only if y has what Leibniz would call a complete individual concept (CIC), a concept that contains within it all predicates of y past, present, and future; the CIC serves to individuate every finite substance out from the infinity of other finite substances, being the sum of the qualitative properties inherent within its given object. The complete individual concept of a substance is thus equivalent to the essence of the substance as it might be known by a omniscient being.