  The Epicureans are epistemological realists who postulate that we naturally form conceptions corresponding to types of object and property in our experience. The metaphysical necessity of property-property pairing explains why the best test of a conditional or of a particular sign is that it is inconceivable that the antecedent exists and the consequent does not (§4, xxxiii 1–7), why someone making a sign-inference on the basis of the evident “locks away in inconceivability” the proposition that things hereabouts are such, but things elsewhere are not such (§1, xv 37–39).