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“An early theoretical use of the concept occurs in Malebranche [Treatise on Nature and Grace] 1674, I, §13; [Dialogues on metaphysics] 1688, 2, 3: Our world is one among the many possible worlds that God could have created. This world, which he did create, differs from the others by the greater simplicity of its laws. The concept is also important in Leibniz’s philosophy. In the Theodicy 1710 he argues that our actual world, having been created by God, a being with all perfections, is the best of all the worlds that he might have created, i.e., the best possible world.
“When explaining the modal concepts of possibility, contingency and necessity, the following pattern of explanation, originating with Leibniz, has proved to be very attractive: A proposition p is necessary if p is true in all possible worlds; a proposition p is possible if p is true in some possible world. The advantage of this pattern of analysis is that the relations between modal concepts are, in a sense, reduced to the relations between the concepts of ‘all’ and ‘some,’ which are accounted for in modern predicate logic.
“Many contemporary philosophers and logicians find the idiom of possible worlds convenient but regard it as a figurative mode of expression only, whilst others, among whom David Lewis is foremost, argue that there is a plurality of worlds.”
— The Penguin Dictionary of Philosophy, 482-483.