 Putnam actually uses a number of different arguments to establish the conclusion above. Of prime concern to realists, as Taylor (2006) emphasises, is the argument based on Gödel’s Completeness Theorem, GCT. For, following Lewis [Lewis, 1984], realists might concede to Putnam that they cannot single out the intended model or distinguish it from various ersatz models, but argue that this is not necessary since it suffices that an intended model exists, even if we cannot specify it. This response does not answer the GCT argument, however. For this argument purports to prove directly that an ideal theory of the world could not be false, a conclusion flatly inconsistent with realism. See the supplementary document  The Model-Theoretic Argument and the Completeness Theorem  for an outline of this argument. 