Full Idea
Goodman has shown that no purely formal criterion can distinguish arguments that are intuitively sound inductive arguments for unsound ones: for every sound one there is an unsound one of the same form. The predicates in the argument make the difference.
Gist of Idea
Goodman showed that every sound inductive argument has an unsound one of the same form
Source
report of Nelson Goodman (Fact, Fiction and Forecast (4th ed) [1954]) by Hilary Putnam - Why there isn't a ready-made world 'Causation'
Book Reference
Putnam,Hilary: 'Realism and Reason: Papers vol 3' [CUP 1985], p.214
A Reaction
This is to swallow grue whole. I think a bit more chewing is called for. By this date Putnam strikes me as a crazy relativist who has lost his grip on the world. Note the word 'formal' - but Putnam seems to think the argument is important.