 He casts the issue as follows. The skeptic argues that a wise man must retreat to skepticism since nothing can be known. This inability is due to the fact that knowledge of a truth—at least as understood by certain Stoics—is only possible if that truth could not possibly be caused to appear mentally by something different than what it is in fact caused by. For example, if an internal mental image or concept of a tree’s being beside a house could be caused by a dream, then the tree’s being beside the house cannot be known, even if the tree is in fact beside the house. With these stringent causal requirements, it is unsurprising to find that Academic Skeptics take the line they do: since no appearance meets this strict standard, they argue, it follows that nothing at all can be known.