Best known for his moral and political philosophy, Thomas Hobbes sets forth a new theory of distributive or social justice. In Leviathan, distributive justice is understood as a social contract theory, which is meant to elevate human beings out of the state of nature. Justice is a matter of keeping covenants, and the only way to ensure that covenants are kept is to create a Leviathan—the absolute sovereign, to whom the people have turned over their autonomy, freedom, and power.
The focus here is on Locke’s epistemology. (A brief account of his political theory is offered in 10.1.) Locke argues for an empirically grounded theory of knowledge, and lays the foundation for such by way of his arguments against innate ideas. He goes on to discuss the formation of ideas, and then their distinctions in terms of primary and secondary qualities.
Like Descartes, Locke attempts to defeat skepticism, but he has been criticized for a view that actually leads to this position. After all, if we have access—direct access—only to our sensations, then we have no guarantee that our perceptions mirror reality. Locke, however, argues that we can believe in the reality of external objects, and our ideas’ likeness to them, because it is unreasonable to think there is no cause of our ideas.
Like Descartes and Locke, Berkeley is interested in refuting skepticism. His conclusion, however, strikes many as counterintuitive. Berkeley denies the reality of material objects, which is to say, he denies the reality of mind-independent entities. To exist unperceived, Berkeley argues, is logically absurd.
Hume’s best-known contribution to metaphysics is to cast down on one of its central concepts, causation. He first distinguishes between two ways we arrive at judgments: relations of ideas and matters of fact. The former provides us with neither knowledge of the world or evidence of the reality of causal relations, and the latter utilizes but cannot justify the concept of causation. One problem is that we do not have any sensible impression of causation. Another is that induction presupposes causation—the very concept it attempts to justify.
Most closely associated with advancing a pantheistic view of God in his Ethics, Spinoza took seriously Descartes’s recommendation about presenting ideas in a geometrical order. At the heart of his argument for God’s nature is his analysis of the concept of substance. Where Descartes argues for two substances, Spinoza claims there is necessarily only one, God.
Leibniz, an impressive polymath, also developed a theory of substance that forms the heart of his metaphysics. Rather than there being two substances (Descartes) or one (Spinoza), Leibniz argues for an infinite number. Since a substance must be simple, it cannot have parts. Moreover, because material objects are extended, they can be divided, and so cannot be simple. A further consequence is that they are not, then, fundamental substance. They are, however, aggregates of simple substances. Simple substances, or monads, are neither extended nor material. Instead, they are mind-like entities.
Monads behave according to a pre-established harmony laid down by God from eternity. They are, in fact, creations of God, and can be annihilated by God alone. Each monad is distinct from every other, but work in consort according to this pre-established harmony. This reflects a further feature of Leibniz’s metaphysics. Since God would choose only the best possible world, this world is, in fact, the best of all possible worlds.