“In everyday language it very frequently happens that the same word has different modes of signification — and so belongs to different symbols — or that two words that have different modes of signification are employed in propositions in what is superficially the same way. Thus the word 'is' figures as the copula, as a sign for identity, and as an expression for existence; 'exist' figures as an intransitive verb like 'go', and 'identical' as an adjective; we speak of something, but also of something's happening. (In the proposition, 'Green is green'— where the first word is the proper name of a person and the last an adjective — these words do not merely have different meanings: they are different symbols.)”