(B) applies only in cases in which S thinks he is seeing, or smelling, or in some other way perceiving something to have F; it does not imply that in such conditions there actually is something that has F, or that it is evident to S that there is something that has F. It implies merely that it is reasonable for S to believe this. Since reasonability is not a sufficiently strong epistemic condition for knowledge, this would not explain how knowledge of the external world is possible. But the next principle may seem to explain this.