In describing a totally unsupportable positive hypothesis as "impossible," we are speaking of a category known as epistemological possibility. This category should not be confused with metaphysical possibility, which will be discussed in a later subsection. As might be guessed from the meanings of "epistemology" and "metaphysics" (pp. 1.2:5-7), epistemological possibility pertains to one's knowledge about a given hypothesis or proposition, while metaphysical possibility pertains to the nature of an entity (more specifically, to those actions or states that are consistent with the nature of an acting entity). The following two assertions, for example, refer to epistemological possibility.

• "There is no possibility that there are gremlins in the Oval Office."
• "Based on a CNN report, it is possible that the president is in the Oval Office at this moment."

On the other hand, it is metaphysically possible for human beings to make mistakes about such matters as the Oval Office, the president, and "gremlins"—that is, humans are fallible by nature. If we clearly distinguish between these two types of possibility (), we can understand readily why gremlins, alien mind control, and other unsupportable hypotheses should be rejected as epistemologically impossible.     

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