Rationality and Time

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One of Parfit's arguments is as follows: self-interest theorists consider the differences between different persons at the same time as significant in terms of rationality, but do not consider the difference between the same person at the different times to be as significant. But if, as Parfit argues, a reductionist theory of personal identity holds, then the difference between different persons at the same time is more like the difference between the same persons at different times. So, if non-reductionism is true, self-interest theorists are inconsistent in viewing spatial relations as significant but temporal relations insignificant. Thus, the foundations of the self-interest theory are undermined by non-reductionism, which lends support to the present-aim theory of rationality, the critical version of which Parfit favours.

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