 Leaving aside now the question of whether there can be contingent propositions that are knowable a priori, there seems to be a difference within the class of necessarily true propositions that can be known, or justifiably believed, a priori. The propositions expressed by  10a–14a  seem different from the propositions expressed by  1a–8a.  Each of the latter propositions seem to be analytic, that is, in each case the sentence that expresses the proposition will express a logical truth if relevant terms and expressions in it are replaced by appropriate synonyms. So, for instance, “all vixens are female” will express a logical truth of the form if \(A \amp B\), then A if we substitute for “vixen”, “female fox”, for it will then say: if something is a female fox, it is a female. But no substitution of synonyms for terms or expressions in  10a–14a  will yield a logical truth. The propositions expressed by  10a–14a  are often said to be synthetic a priori propositions because they are not analytic (they are not true in virtue of their meaning, as it is sometimes put) but are a priori knowable and justifiable. 