The judgment that an artist is great always rests on assessments of the work the artist has produced  A series of great works is the only indicator of greatness
Ttherefore, to say that an artist is great is just to summarize the quality of his or her known works, and the artist' s greatness can provide no basis for predicting the quality of the artist' s unknown or future works
Unless a person displays cold symptoms, it cannot properly be said that the person has a cold. But each of the symptoms of a cold is also the symptom of some other disease. Ttherefore, one can never be certain that a person has a cold.