Kinds Of Impossibilities

Do you expect someone seated on a Wellsian Time Machine or a Time Patrol timecycle to appear in front of you? Or a man just standing without benefit of any kind of vehicle like Poul Anderson's Jack Havig or some other fictional time travellers? Of course such an event is not going to happen but why not? It is logically possible, i.e., not logically impossible, i.e., not self-contradictory - like a square triangle or an exception to the proposition that all white men are men.

For clarity of thinking, it is important to understand the difference between an empirical question and a conceptual question. It is possible that there is a very small red triangle hidden in this room. We can respond to the possibility by searching the room very thoroughly. It is impossible that there is a square triangle anywhere in the room. We respond to this impossibility not by looking anywhere in the room but by analyzing the contradiction in the phrase, "square triangle."

In addition to logical impossibilities, there are also physical impossibilities. I cannot jump to the moon although there does not seem to be any contradiction in the phrase, "...jump to the moon." This might be partly a matter of linguistic precision. If I were defined very precisely as an organism with a limited range of physical capabilities that explicitly excluded the ability to jump to the moon, then it would become logically contradictory to state that this organism that is unable to jump to the moon is able to jump to the moon. But that still seems to leave open the logical possibility that, without undergoing any detectable enhancement in its physical abilities, an organism might nevertheless "miraculously" jump to the moon. But we would respond to such events if they occurred.

Knowing both what is logically impossible and also what is physically impossible delimits what could have happened even when we do not know what did happen. And I think that the main role of the logically impossible is to delimit what can happen, not to imply that anything that is merely logically possible might happen. Until further notice, the arrival of travellers from other times must be regarded as physically impossible.