 The modal notions considered in the last paragraph are not obviously epistemological. On the face it, we are not reporting a fact about what is or can be known or believed by anyone when we say that life could have failed to develop. But there is also a family of modal concepts that are clearly epistemological. These are the notions we employ when we say things like ‘Fred must have stolen the book (the evidence shows conclusively that he did it),’ or ‘Mary cannot be in London (she would have called me).’ These modal utterances seem to make claims about what the available evidence shows, or about which scenarios can be ruled out on the basis of the evidence. More formally, we can say that a proposition \(P\) is epistemically necessary for an agent A just in case the empirical evidence \(A\) possesses and ideal reasoning (i.e., reasoning unrestricted by cognitive limitations) are sufficient to rule out \({\sim}P\). This notion of epistemic necessity is agent-relative: one and the same claim can be epistemically necessary for one agent, but not for another agent with less empirical evidence. We obtain a notion of epistemic necessity of particular philosophical interest by focusing on a limiting case, namely that of a possible agent with no empirical evidence  whatsoever.[1]  A proposition \(P\) is epistemically necessary for such an agent just in case ideal reasoning alone, unaided by empirical evidence, is sufficient to rule out \({\sim}P\). A proposition that meets this condition can be called a priori in at least one sense of this term, or we can call it simply epistemically necessary (without relativization to an agent). Propositions that are not a priori are called a  posteriori.[2]