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52 that presentational immediacy is primitive, and that causal efficacy is the sophisticated derivative. This is a complete inversion of the evidence. So far as Hume’s own teaching is concerned, there is, of course, another alternative: it is that Hume’s disciples have misinterpreted Hume’s final position. On this hypothesis, his final appeal to ‘practice’ is an appeal against the adequacy of the then current metaphysical categories as interpretive of obvious experience. This theory about Hume’s own beliefs is in my opinion improbable: but, apart from Hume’s own estimate of his philosophical achievement, it is in this sense that we must reverence him as one of the greatest of philosophers.

The conclusion of this argument is that the intervention of any sense-datum in the actual world cannot be expressed in any simple way, such as mere qualification of a region of space, or alternatively as the mere qualification of a state of mind. The sense-data, required for immediate sense-perception, enter into experience in virtue of the efficacy of the environment. This environment includes the bodily organs. For example, in the case of hearing sound the physical waves have entered the ears, and the agitations of the nerves