Consider the following two propositions:

A: There exists extraterrestrial life somewhere in the universe.

B: There does not exist extraterrestrial life anywhere in the universe.

B is just the converse of A. However, B is falsifiable (simply by finding life beyond earth), whereas A is not (because it is impossible to check every location in the universe for life, or to be sure that we haven't missed life in a place we've already checked).

So it would seem that by Popper's falsifiability criterion, B is a scientific statement whereas A is not. But this feels misguided, since both statements are so closely related and don't reference any vague pseudoscience. Both statements feel scientific since they are reasonably precise and could be investigated through empirical methods.

So several questions come to mind:

  1. Would Popper conclude that B is scientific and A is not?
  2. Is this a defect in Popper's criterion of falsifiability?
  3. Have modern philosophers of science refined Popper's ideas in order to better address this problem?
  • "B is falsifiable, A is not" - exactly the opposite.
    – gnasher729
    Feb 6 at 18:08
  • @gnasher729 Suppose we find life on Mars (or anywhere). Then this finding falsifies B.
    – WillG
    Feb 6 at 18:09
  • I think this issue is just a natural consequence of Popper trying to interpret scientific theories as logical statements involving the universal quantifier and thus concluding that the only sort of valid test is looking for a single counterexample that can falsify that statement. As for #3 on criticisms of this as a way of thinking about philosophy of science, see my comment here, also the "criticisms" section of this page from this site.
    – Hypnosifl
    Feb 6 at 19:32
  • 2
    Popper's falsifiability applies to theories, not to individual statements. Theories may include all sorts of statements, falsifiable or not individually, but they must not be idle/redundant and they must produce falsifiable consequences. "There exists extraterrestrial life somewhere in the universe" is too vague to be a scientific theory by itself. One has to be much more specific as to where it exists or doesn't exist to make it a scientific claim within some theory.
    – Conifold
    Feb 7 at 0:00

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