In a recent blog entry, theistic philosopher William Vallicella criticizes a statement made by psychologist Paul Vitz, in which Vitz asserted that it is “intrinsically impossible” to “prove the non-existence of anything.” As Vallicelli correctly points out:

“But surely there are things whose nonexistence can be proven. The nonexistence of a round square can be proven a priori by simply noting that something that is both round and nonround cannot exist.”

What Vallicelli writes is consistent with my own essay on the subject, where I made the following observation.

Indeed, there are actually two ways to prove the nonexistence of something. One way is to prove that it cannot exist because it leads to contradictions (e.g., square circles, married bachelors, etc.). …The other way to prove the nonexistence of something is, in the words of Keith Parsons, “by carefully looking and seeing.”

I could not agree with Vallicelli more when he concludes that Vitz’s assertion is “plainly false.”

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