In this thesis, "time travel" means travelling backward to the past, which is a problem studied in many approaches. But I study only the problem of logical possibility. The purposes of the thesis are to study the concept of time travel; analyze the paradoxes of time travel discussed in philosophy; and evaluate the possibility of time travel. Firstly, I study the paradoxical problems of time travel, which have two main forms: Grandfather paradox and Knowledge paradox. Grandfather paradox showed that time travel entails self-contradictory situations, such as killing grandfather before begetting one's own father-hence it is itself logically impossible. Lewis rejected this entailment by indicating that these situations will be foiled by coincidences. But I agree with Horwich's proposal that the coincidences that make killing-grandfather attempts continuously fail provide empirical reason to infer that time travel entails the improbable situations. Hence time travel is unlikely to occur in the actual world. Knowledge paradox demonstrated that time travel entails bizarre situations, self-parenting of time traveller, creation ex-nihilo of physical objects and deriving knowledge without problem-solving process. But I show that this paradox does not imply any self-contradiction-hence time travel is not logically impossible. But according to Horwich's reasoning we also have empirical reasons, from what known to be true in our world, to infer that time travel is improbable in the actual world. Secondly, I consider the proposition of Deutsch and Lockwood on using many-universe interpretation of quantum mechanics to dissolve the paradoxes of time travel. Though it could avoid the paradoxical problems I point out that this idea is, due to the problem of infinite parallel universes in such interpretation, epistemologically impossible.
