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HIS paper presents a partial analysis of perceptual know-ledge, an analysis that will, I hope, lay a foundation for a general theory of knowing. Like an earlier theory I proposed,' the envisaged theory would seek to explicate the concept of knowledge by reference to the causal processes that produce (or sustain) belief. Unlike the earlier theory, however, it would abandon the requirement that a knower's belief that p be causally connected with the fact, or state of affairs, that p.What kinds of causal processes or mechanisms must be responsible for a belief if that belief is to count as knowledge? They must be mechanisms that are, in an appropriate sense, "reliable." Roughly, a cognitive mechanism or process is reliable if it not only produces true beliefs in actual situations, but would produce true beliefs, or at least inhibit false beliefs, in relevant counterfactual situations. The theory of knowledge I envisage, then, would contain an important counterfactual component.To be reliable, a cognitive mechanism must enable a person to discriminate or differentiate between incompatible states of affairs. It must operate in such a way that incompatible states of the world would generate different cognitive responses. Perceptual mechanisms illustrate this clearly. A perceptual mechanism is reliable to the extent that contrary features of the environment (e.g., an object's being red, versus its being yellow) would produce contrary perceptual states of the organism, which would, in turn, produce suitably
