It is generally believed that, for a one-off Prisoner's Dilemma game, it is logical to defect. However, both players cooperating is apparently a better choice than both defecting, hence the dilemma. In this paper, by resorting to Ramsey's Test, Kripke's possible world semantics, and Stalnaker/Lewis-style account of conditionals, I show that the first horn of the Prisoner's Dilemma is an unsound argument. It originates from failing to differentiate between a possible world and a possible set of possible worlds and failing to observe that the set of accessible possible worlds associated with a possible world in general varies from conditional to conditional. This phenomenon can also be illustrated in terms of the recently developed hi-world semantics. Moreover, a meta-argument is constructed to establish the non-existence of a logical argument for defection.