  Even the individualistic criterion raises considerable complications and difficulties. We speak of a disposition to assent (or dissent) in response to a pattern of stimulation but this is not quite accurate. Such a pattern, a complete list of which sensory nerves are firing, and in which order, will hardly ever repeat itself. So what we need is, rather, the idea of a correlation of a response with a type of stimulation  pattern.[1] But the relevant idea of a type here is complicated. The physical resemblance of two stimulation patterns, what Quine calls receptual similarity, is not enough to make them constitute events of the same type, in the relevant sense; two such patterns may resemble each other very closely yet lead to quite different responses. (Two occasions on which I am driving a car may be almost identical in terms of my stimulation patterns, except that on one occasion I see a red light and on the other I see a green light. As far as my response goes, that small difference outweighs all the similarities.) What is wanted is a more complex notion which Quine calls perceptual similarity. Very roughly, two stimulation patterns count as similar (for an animal, at a time) if they tend to lead to the same  response.[2]