 That is, in the close worlds to the actual world in which not-p holds, S does not believe p. The actual world is one’s situation as it is when one arrives at the belief p. BT requires that in all nearby not-p worlds S fails to believe p. (The semantics of subjunctive conditionals is clarified in Stalnaker 1968, Lewis 1973, and modified by Nozick 1981 note 8.) On Dretske’s view knowing p is roughly a matter of having a reason R for believing p which meets the following condition (‘CR’ for conclusive reason):