I enjoyed this thought from Eric Lonergan's talk here at Words by the Water, which has the feel of a brain teaser.

If you think of personhood as something that changes, as seems quite natural - I'm not quite the same person as I was yesterday, or will be tomorrow - then life can be viewed as a continuous act of altruism. For yesterday's self should have acted to look after today's, and did, more or less, as I am here. And today, I must act to look after tomorrow's.