 To supplement these examples, it would be nice to give characterizations of the three notions that are free from controversy. That is easier said than done. Nonetheless, we offer some characterizations that are relatively uncontroversial. Metaphysical possibility is often taken as a primitive notion that figures into the idea of a physical possibility: a proposition is physically possible if and only if it is metaphysically compossible with the laws of physics. (Other nomological possibilities, such as chemical or biological possibility, can be understood similarly.) Assuming that the notion of a  logical truth  is understood, then the logical necessities are simply the logical truths, so that the logical possibilities are those things whose negations are not logical truths.