be a colour; if by the ears, a sound; if by the palate, a taste; and so of the other senses.” Thus in asserting the lack of perception of causality, he implicitly presupposes it. For what is the meaning of ‘by’ in ‘by the eyes,’ ‘by the ears,’ ‘by the palate’? His argument presupposes that sense-data, functioning in presentational immediacy, are ‘given’ by reason of ‘eyes,’ ‘ears,’ ‘palates’ functioning in causal efficacy. Otherwise his argument is involved in a vicious regress. For it must begin again over eyes, ears, palates; also it must explain the meaning of ‘by’ and ‘must’ in a sense which does not destroy his argument.
This double reference is the basis of the whole physiological doctrine of perception. The details of this doctrine are, in this discussion, philosophically irrelevant. Hume with the clarity of genius states the fundamental point, that sense-data functioning in an act of experience demonstrate that they are given by the causal efficacy of actual bodily organs. He refers to this causal efficacy as a component in direct perception, Hume’s argument first tacitly presupposes the two modes of perception, and then tacitly assumes that presentational immediacy is the only mode. Also Hume’s followers in developing his doctrine presuppose
