 An additional problem is that the criteria can pull in different directions. To illustrate, consider Reid’s argument for life on other planets  (Example 2).  Stebbing (1933) finds Reid’s argument “suggestive” and “not unplausible” because the conclusion is weak (G4), while Mill (1843/1930) appears to reject the argument on account of our vast ignorance of properties that might be relevant (G3).