The purpose of this article is to read Heidegger's critique of modern science -especially in What Is a Thing? - as evolving from ontological issues that preoccupied Heidegger in the period after the publication of Being and Time. The main issues at stake are formal ontology and its connection with mathematics and modern mathematical physics, and the distinction between formal and regional ontology. The connection between these issues constitutes Heidegger's understanding of mathematics. An exposition of Heidegger's notion of the "mathematical" can help us uncover his assumptions in his critique of modern science, and give us new possibilities for evaluating it.