This thesis articulates a novel interpretation of Heidegger's explication of the being (Seins) of gear (Zeugs) in §15 of his masterwork Being and Time (1927/2006) and develops and applies the position attributed to Heidegger to explain three phenomena of unreflective action discussed in recent literature and articulate a partial Heideggerian ecological metaphysics. Since §15 of BT explicates the being of gear, Part 1 expounds Heidegger's concept of the 'being' (Seins) of beings (Seienden) and two issues raised in the 'preliminary methodological remark' in §15 of BT regarding explicating being. §1.1 interprets the being (Sein) or synonymously constitution of being (Seinsverfassung) of a being (Seienden) as a regional essence: a property unifying a region (Region), district (Bezirk), or subject-area (Sachgebiet) - a highly general ('regional') class of entities. Although Heidegger posits two components of the being of a being, viz. material-content (Sachhaltigkeit, Sachgehalt) and mode-of-being (Seinsart) or way-of-being (Seinsweise, Weise des Seins, Weise zu sein) (1927/1975, 321), the unclarity of this distinction means that it does not figure prominently herein. §1.2 addresses Heidegger's distinction between ontological and ontic investigations and his notion of 'modes of access' (Zugangsarten, Zugangsweisen). Part 2 expounds §15 of BT's explication of the being of gear. §2.1 analyses Heidegger's two necessary and sufficient conditions for being gear and three core basic concepts (Grundbegriffe) enabling comprehension of these conditions and therewith a foundational comprehension of gear. Heidegger explicates the being of gear through content of unreflectively purposeful, non-intersubjective intentional states. I term such states 'mundane concern', which is almost synonymous with Hubert Dreyfus's term 'absorbed coping' (1991, 69). Heidegger's explication highlights around-for references (Um-zu-Verweisungen) as the peculiar species of property figuring in mundanely concernful intentional content. §2.2 clarifies Heidegger's position on the relationship between to-hand-ness (Zuhandenheit) and extantness (Vorhandenheit) in the narrow sense: two of Heidegger's most widely discussed concepts. I reject Kris McDaniel's recent reading of Heidegger as affirming that nothing could be both to-hand and extant simultaneously (McDaniel 2012). Part 3 develops and applies Heidegger's phenomenology of mundane concern. §3.1 explains the phenomena of situational holism, situated normativity, and mundanely concernful prospective control. §3.2 undertakes the metaphysical accommodation of around-for references, which §3.1 posited as featuring prominently within mundanely concernful intentional content. This thesis thus contributes not only to Heidegger scholarship, but also to contemporary debates within the philosophy of action and cognitive science.