Our past is available for imaginative construction. If our ancestors were given different goals and dice rolls, we would have a different present. We need not only speculate on future hypotheses, we can also counterfactually think through alternative histories. For instance, "what would our present be like if the Cuban missile crisis hadn't happened?" These imagined realities are possible worlds: worlds which are coherent and logically sensible but just so happen to not be the actual world we find ourselves in.
