One of the most famous objections to time travel is the 'grandfather paradox' - that is, suppose I go back in time to kill my grandfather before my parents are born. If I kill my grandfather, I could not have existed - therefore I would not be able to go back in time and kill my grandfather, and so on. In order to address such objections, many philosophers have drawn a distinction between changing the past and affecting the past.

Before addressing the distinction, it is necessary to formulate a general conception of what time is. The two main conceptions are the Parmenidean concept of time, which holds that reality is unchanging and eternal (the static view) and the Heraclitean concept, which holds that the world is a never-ending process of change (the passage view). Under the static view, the past and future are just as real as...