Abstract: Arthur Prior and Carew Meredith cooperated on the formulation of several systems of logic. The cooperation was so close that on the basis of their joint work, they are both considered to be precursors of possible worlds semantics. However, their concept of possible worlds, their understanding of the relevant formal representations and indeed their general approach to modal logic considerably differed. These differences should be pointed out in order to more precisely appreciate the contribution of each of these authors. To neglect the differences could cause the misinterpretation of Meredith’s and Prior’s work. On the one hand, it might cause corruption of Meredith’s system of logic and lead to paradoxes, as Prior pointed out in ‘Modal Logic with Functorial Variables and a Contingent Constant’. On the other hand, considering Prior as a mere follower of Meredith could cause an underestimation of Prior’s originality and contribution to this field.