John Locke (1632-1704): though he believed that the material of knowledge is supplied by sense perception and introspection (empiricism), he also believed we could know about things outside of sensations (metaphysics). He thought we could reason about spiritual and supernatural truths; he disliked authoritarianism and favored toleration within limits.
Theory of Knowledge: In order to avoid skepticism and to provide a firmer basis for science, we should first focus on the study of knowledge (Essay concerning Human Understanding, 1690) to determine the extent, powers, limitations of human understanding: with what is our understanding fit to deal? Set the limits of proper skepticism: origins, certainty, extent of human knowledge; grounds and degrees of belief, assent, opinions; and limit our assent to the evidence.
Thus his method of inquiry includes three tasks: first, to inquire into the origin of ideas (a psychological inquiry); second, to show what knowledge the mind has by those ideas and the certainty, evidence, and extent of that knowledge; and third, to inquire into the nature and grounds of faith or opinion (the second and third tasks are epistemological). [Problem: this seems to imply that we will distinguish those things with which the mind is capable of dealing from those with which it is incapable of dealing; but how can we do this without passing beyond the scope of the mind?] Locke is not concerned with a physical or physiological explanation of understanding. Bks. 1-3 inquire into the origin of ideas; Bk 4 deals with knowledge and the grounds for beliefs or opinion.
There are no innate ideas if what is meant by that term is that everyone is born knowing certain things. For innate ideas must be ideas (i.e., known), otherwise they are not ideas; so the claim that they are known virtually (unknown by children and idiots) is meaningless. If innate ideas are really innate capacities to recognize truths, then every truth we learn would be innate (which simply cannot be correct). Even if there were universal agreement about certain claims, that would not prove their innateness. No matter, because there is no proposition about which there is universal agreement, even regarding God's existence or moral beliefs. The principles that seem to be agreed upon (e.g., what is, is; or something cannot be and not be at the same time) are explainable in terms of education and experience. Only ideas grounded in experience provide knowledge (empiricist principle). Experience includes sensation (how external things affect us) and reflection (the mind's consideration of its own operations, such as perceiving, thinking, believing, doubting, willing).
An idea is whatever is the object of our understanding. What we know are our ideas and the relations between them, and we know of the things to which those ideas refer indirectly insofar as ideas represent them. [But this implies that we can know some things directly insofar as representational realism assumes that there is something real to which ideas refer.] Ideas represent things, words represent ideas. Therefore to speak about something is to refer indirectly to the thing through the idea. "Idea" refers to either the content of a sense perception (e.g., the experience of pain), a belief or concept (as when we talk about Locke's ideas), or a mental image (e.g., a memory of being in pain).
Simple ideas are passively received, unresolvable further, bare observation or introspection. Simple ideas of sensation (one sense) include cold, hard, solid, white; (more than one sense): space, motion, figure. Simple ideas of reflection: thinking, willing. Simple ideas of sensation and reflection combined: pleasure, pain, power, existence, unity, succession. (God has associated the sensation of pleasure and pain with some of our ideas to guide us in acting.) Complex ideas (of substances, modes, relations) are actively formed by the mind when it repeats, compares, or unites simple ideas: beauty, gratitude, man, army, universe, wife, sugar. Classification of complex ideas can be done based on objects (modes, substances, and relations) or based on the mind's activities (combinations of simple ideas; abstractions--that is, separating common elements among various simple ideas; and comparisons of two ideas with one another--ideas of relations, like daughter, whiter, smaller). Modes depend on substances, and substances are combinations of simple ideas that represent independently existing things. Simple modes are various combinations of the same idea (space, time, number, infinity); mixed modes are combinations of simple ideas of different kinds (e.g., triumph, murder, drunkenness).
A quality is the power in a thing to affect another thing, including the power in a thing to produce ideas in the mind. Primary qualities are powers inseparable from body regardless of changes (solidity, extension, figure, motion/rest, number)--the bulk, figure, number, situation, and motion/rest of the solid parts ("corpuscles") of bodies; such ideas resemble the ways that things are in themselves. Secondary qualities are powers to cause in us ideas (hot, hard, sweet) that don't resemble the things themselves but that are produced by primary qualities through the effects of insensible particles on the senses. (In contrast to the relation between primary qualities and the ideas they cause, there is no apparent connection between secondary qualities and the ideas they cause. Experience shows that sickness, illusion, and change of condition--e.g., grinding up an almond causes it to lose some of its sensible characteristics but not others.)
Ideas are signs of things, words are signs of ideas. We don't know things immediately but mediately, by means of ideas. Substances exist independently of our ideas, which are privately known and shared in words (representational realism). Even though we know only our ideas, we also know that external objects cause us to have ideas because we do not control how and when we get ideas. Besides, practical experience ("the ordinary affairs of life") requires that we assume external objects.