248 An Introduction to Immortalist Morality radically alter the minds and bodies of those who desire it. For instance we could imagine ‘brain refresher drugs’ which prevented brains from becoming too inflexible. The people living  in  the  far  future  might  be  able  to  alter  their  bodies and personalities as easily as the people of today change their clothes. The fact that some people living today get tired of life is more likely to be a practical, biological problem than a philosophical one. IDENTITY Another  philosophical  concern  emerges.  An  individual might worry that if he lived long enough he would cease to be ‘himself’ and become someone else. After all, exactly what is the ‘self’? Astronomer Martin Rees recently expressed this worry: I’m reconciled to extinction – losing all consciousness as well as rotting away physically. Indeed, I think we should welcome the transience of our lives. Individual immortality  would  be  deleterious  for  life’s  further development unless we could transform ourselves, men- tally and physically, into something so different from our present state that the transformed entities wouldn’t really still be ‘us’. If technology allowed me to transcend these limitations, I would only be the same person in the sense that I would retain some memories of early life. But even over present life spans it is not clear how much continuity of personality is really preserved. Each of us is a ‘bundle of sensations’ somehow woven together as a continuous thread or ‘world line’. [4] The idea that we are just a ‘bundle of sensations’ dates back to philosopher David Hume, but other philosophers such as