As a contribution towards clearing the ground for a new phenomenological evaluation of the essence of science, in this paper I present a critique of Heidegger's argument in Being and Time for the priority of Zuhandenheit to Vorhandenheit. I argue that Heidegger's notion of presence-at-hand is incoherent, conflating Husserl and Descartes, and that this general analysis has serious phenomenological flaws. Contrary to Heidegger, I maintain that there is a form of exploratory, theoretical activity including causal inquiry which is prior to the type of practical activity entailed by readiness-to-hand. Consideration of this exploratory activity points us in the direction of an alternate conception of the genesis and essence of theory.