- By 'the unity of psychology' I mean something one might also express by saying that the psychology of human beings is part of the psychology of animals generally.
- Perhaps there are several different ways of trying to trace out the ramifications of the idea that psychology is one. A central consideration, I think, is likely to be some sort of principle of continuity up and down the scale of nature. The idea would be that up and down the scale of animated or ensouled things ('psyched up' beings, empsucha) there are always psychological continuities, never any strict discontinuity. If human beings can get angry, can want to get ahead in life, can see an illusion, can develop an Oedipus complex, then so can some lower animal do either the very same thing, something similar, or at least something analogous.
- On my reconstruction, then, Descartes' confidence that only human animals1 are capable of consciousness rests ultimately on the idea that only human beings are capable of doubting propositions and hence only human animals2 are capable of a self-transparent 'inner' or mental life.
- Of course this reasoning will not do. At least some non-human animals are capable of suffering pain. In some cases their pain states model pain states in human beings. (That we believe this to be true is shown by certain experimental uses we make of non-human animals to test drugs, food and cosmetics intended eventually for human beings.) If their pain states were not genuine mental states, but only mechanical ones, as Descartes supposes, it would always be inappropriate to feel compassion for the sufferings of non-human animals. But it is not always inappropriate. Therefore it is false that the pain states of non-human animals are not genuinely mental, but only mechanical states. If such animals lack, as I suppose many of them do, the capacity to doubt propositions, then the capacity to doubt propositions is not a prerequisite for having a genuine mental life.
- My own best effort to reconstruct Cartesian reasoning for the conclusion that non-human animals are non-conscious automata thus yields nothing more persuasive than the efforts of Vendler and Malcolm. I conclude that Descartes has given us no good reason to reject the Principle of Psycho- logical Continuity and no plausible basis for denying the unity of psychology.
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