Kant's writings on logic illustrate the comparison argument about truth, which goes as follows. A truth-bearer p is true if and only if it corresponds, or it agrees, with a portion of reality: the object(s), state(s) of affairs, or event(s) p is about. In order to know whether p agrees with that portion of reality, one must check if that portion of reality is as p states. Using the terms of the comparison argument, one must compare p with that portion (...) of reality. This is impossible, because the only knowledge of reality we can have is in the form propositions, beliefs, or judgments, whose agreement with reality is as much in need of justification as the agreement of p with reality. Therefore, it is impossible to know which truth-bearers are true. This paper reconstructs Kant's version of the comparison argument. It is argued that, according to Kant, the argument is sound only under the assumption of transcendental realism. Transcendental idealism avoids the sceptical consequences of the comparison argument. (shrink)