A possible world is a situation which realizes some possible, though not necessarily actual, state of affairs, such as Caesar avoiding assassination, or Germany winning the Second World War. The idea of such worlds can be traced back to the philosopher Gottfried Leibniz (1646–1716), but they have come into prominence in contemporary logic as a result of their use in Kripke semantics. What possible worlds are, metaphysically, is a contentious matter. Some take them to be abstract objects, like sets of propositions; some take them to be non-existent objects; some take them to be concrete worlds, just like the actual world, but divorced from it in space and time; some take them to be bodies of information.