<logic, epistemology> what cannot be the case, under any circumstances, is impossible. What is logically impossible is self contradictory; inconsistent with the basic principles of logic itself (to be both human and nonhuman, e.g., is logically impossible). It is convenient for many purposes to recognize types of impossibility weaker than strict logical impossibility. Natural or nomological impossibility is the next strongest generally recognized type: what is nomologically impossible, while it may be logically consistent, is inconsistent with the laws of nature: e.g., it's nomologically impossible (current physics tells us) for anything to travel faster than the speed of light. Practical impossibility is a weaker variety yet: what is practically impossible may be consistent with the laws of nature, but is inconsistent given the circumstances; e.g., it's nomologically possible for a human being to run a four minute mile but it's not practically possible for most of us (given our ages, physiques, and physical conditions) to do so. Contrast: possible. See also: necessary, contingent, actual.
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