I think that time travel is logically possible, but I think that even if it were physically possible no time traveler would be able to go back in time and kill her baby self. My argument has two premises:
- Premise1: A person can do something (is able to do that thing) only if it isn't always true that: if he tried to do it, he would fail.
- Premise 2: It is always true of Suzy, the time traveller, that if she tried to kill the baby who is her younger self, she would fail.
In this post I'm going to defend the first premise. That premise isn't specific to time travelers. It asserts a quite general link between our beliefs about what a person can do and the truth of certain counterfactuals. That is why (1) is worth thinking about quite apart from worries about time travel: it captures an essential feature of what we mean when we say that people are able to do things even if they don't do them. It partly defines how we understand 'can' when we make choices: that is, when we choose to do one thing from among the other things we can do.