Full Idea
Assume that we know to be true propositions of the form 'There are exactly x prime numbers between y and z', and synthetic a priori truths about Platonic objects are delivered to us on a silver platter.
Gist of Idea
If we know truths about prime numbers, we seem to have synthetic a priori knowledge of Platonic objects
Source
José A. Benardete (Metaphysics: the logical approach [1989], Ch.18)
Book Reference
Benardete,José A.: 'Metaphysics: The Logical Approach' [OUP 1989], p.139