What was Frege trying to prove? A response to Jeshion

Why did Frege look for the foundations of arithmetic in logic? Robin Jeshion has argued against several proposed answers, mine among them, and offered one of her own. In response, I argue that (i) Jeshion's own interpretation does not work: it is unsupported by the text and fails to answer the question; (ii) while it is not my view that Frege is motivated solely by philosophical concerns, his motivation cannot be divorced from his belief that foundations for science must show whether its truths are analytic or synthetic, a priori or a posteriori; (iii) it must (for Frege) be evident from the content of a primitive truth to which of these categories the truth belongs.