  It is worth emphasizing the fact that the definition of the key notion of perceptual similarity is behavioural. It avoids any idea of experience, of awareness, of what strikes the person (or other animal) as more similar to what. It is simply a matter of responses, and thus of behaviour. (Assent is also treated as behavior, as sound-making propensities; see 1975c pp. 87–88). This is in accord with Quine’s insistence on what he takes to be scientific standards of clarity and rigour. One consequence of it is that the notion cannot be invoked to explain behaviour. Quine is under no illusions on this score. (See 1975c, 87, where the point is explicit.) The behavioural account does not explain our understanding of observation sentences; explanation, if possible at all, comes at the neuro-physiological level. What the behavioural account does is to make clear exactly what behaviour constitutes that understanding and, hence, what the neuro-physiological account would have to explain. (It also shows that there is indeed something to be explained.)