 A.N. Prior constructed two systems showing how that can be done, namely the Peircean system (in which (P1) and (P5) are rejected) and the Ockhamist system (in which (P3) is rejected). Formally, each of these systems offers a basis for the rejection of the claim of determinism as expressed in (8). As we shall see in the next section the same can be said about Jan Łukasiewicz’s three-valued semantics, which Prior himself investigated further (see Prior 1953), and which also involves a rejection of (P5). Since Prior, several philosophers have discussed which one of these systems should be accepted, or whether other and more attractive systems dealing with the problem can be constructed. In sections 4 and 5 we shall see how the various solutions to the problem of future contingents can be grouped according to their consequences with respect to (P3) and (P5).