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Full Idea
From the fact that one can think of the sum of seven and five without necessarily thinking of twelve, it by no means follows that the proposition '7+5=12' can be denied without self-contradiction.
Gist of Idea
We can think of 7 and 5 without 12, but it is still a contradiction to deny 7+5=12
Source
comment on Immanuel Kant (Critique of Pure Reason [1781]) by A.J. Ayer - Language,Truth and Logic Ch.4
Book Ref
Ayer,A.J.: 'Language, Truth and Logic' [Penguin 1974], p.104
A Reaction
Kant's claim that arithmetic was synthetic always looked glib and dubious, and this pinpoints an objection very nicely. It appears that the great Kant has confused his epistemology with his ontology.