Here's a quote from Hume: "Nothing, that is distinctly conceivable, implies a contradiction." My question is this: what is the difference between something that is logically a contradiction and something that happens to not be instantiated? For example, ghosts do not exist. Could you explain how the concept of a ghost is not a contradiction? Thanks ^^

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What is the difference between something that is logically a contradiction and something that happens to not be instantiated?

As I think you already suspect, it's the difference between (1) a concept whose instantiation is contrary to the laws of logic or contrary to the logical relations that obtain among concepts; and (2) a concept whose instantiation isn't contrary to logic but only contrary to fact. Examples of (1) include the concepts colorless red object and quadrilateral triangle. Examples of (2) include the concept child of Elizabeth I of England. Concepts of type (1) are unsatisfiable in the strongest sense; concepts of type (2) are merely unsatisfied.

Could you explain how the concept of a ghost is not a contradiction?

Good question. I'm not sure the concept isn't internally contradictory. Can ghosts, by their very nature, interact with matter? Some stories seem to want to answer yes and no. If I recall correctly (it's been a while) the movie Ghost (1990) depicts the rookie ghost struggling to interact with matter well enough to move just a penny, despite the fact that he has no trouble standing on the floor rather than passing right through it. It's as if he can interact with matter only when he's not consciously trying to. I seem to recall that Field of Dreams (1989) has similar inconsistencies. But it may be that both movies can be interpreted as internally consistent if we import enough ad hoc principles of ghostly metaphysics.

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