Full Idea
Sidgwick said self-interest is not self-evidently rational. Unless we invoke a religious idea of the soul, human personality is no more than a succession of continuities in memory and behaviour. In that case, why should anyone favour their future self?
Gist of Idea
Self-interest is not rational, if the self is just a succession of memories and behaviour
Source
report of Henry Sidgwick (The Methods of Ethics (7th edn) [1874]) by John Gray - Seven Types of Atheism 2
Book Reference
Gray,John: 'Seven Types of Atheism' [Penguin 2019], p.39
A Reaction
This sounds like Locke's account of the self, as psychological continuity. We can say that our continuous self is a fiction, the hero of our own narrative. Personally I think of the self as a sustained set of brains structures which change very little.