Idealism

The idealism of Gottfried Wilhem Leibniz (1646–1716) arose largely in response to questions raised by Descartes about the relation between mental substances and physical substances. According to Leibniz, real substances do not and cannot interact, because to be a substance is to be independent of the influences of other substances (but no finite substance is altogether independent of God, who is the ground and cause of all finite substances, including ourselves). Furthermore, Leibniz argued that every genuine substance must be utterly non-composite or simple (i.e., not made of parts), because the ongoing unity and existence of any being made of parts depends on causes outside of the being itself (and such dependence contradicts the very definition of substance). Accordingly Leibniz held that no genuine substance can be material, because matter is essentially composite, which means that matter cannot be substantially or independently real. Leibniz thus concluded that substances must be percipient, or have perceptions, because the only way in which a substance can be utterly simple and yet reflect diversity within itself is through the undivided activity of perception. Leibniz's idealism can be summed up in the proposition that "to be is to be a substance, and to be a substance is to be percipient." For Leibniz, the real world is simply the totality of all such noninteracting ("windowless") and percipient substances (called "monads"), and our experience of the material world is to be explained idealistically: to be a substance is to be percipient, and the perceptions belonging to any one substance accurately reflect the states of all other substances, not because there is any real interaction among substances but because God has ordained a "pre-established harmony" among all finite substances and their perceptions.

If modern philosophy is divided into two main schools of thought—"rationalism" and "empiricism"—then Leibniz is a "rationalist" idealist, while George Berkeley (1685–1753) is an "empiricist" idealist. Berkeley began with John Locke's empiricist premise that the mind does not possess innate ideas but acquires ideas only through sensory experience. Like Locke, Berkeley also held that the mind has immediate or direct perception only of its own ideas. But unlike Locke, Berkeley denied that the mind's immediate perception of its own ideas can give it indirect knowledge of material things outside of it. Berkeley further insisted that "an idea can be like nothing but an idea" (Principles, part 1, section 8), and so we can never know whether the immaterial ideas in our minds resemble or accurately depict material things outside of our minds. Furthermore, Berkeley argued, there is something self-contradictory in the proposition that objects of perception can exist without being perceived. In order to avoid skeptical or altogether absurd conclusions, Berkeley argued, one must abandon belief in the independent existence of material things and become an idealist or "immaterialist." For Berkeley, the ideas that we have of sensible things are not caused in us by independently existing material things; rather, these ideas simply are the sensible things themselves. But sensible things have continued existence—even when we finite minds are not perceiving them—because they continue to exist in the mind of God, whose perception of things not only causes the sensible things to exist but also from time to time causes them to be perceived by us. Thus for Berkeley, our perception of sensible things is nothing other than our perception of ideas in God, and sensible things have an orderly, predictable, and enduring existence because of the wisdom and goodness of God. For Berkeley, then, the immaterialist view of reality not only refutes skepticism but also provides indirect theoretical support for theism. Far from seeking to reduce the real world to the status of "mere" ideas, the real aim of Berkeley's immaterialism is to elevate "mere" ideas to the status of the real world.