In contemporary philosophy of science, ontological reductionism, or the claim that everything that exists in the world is something physical, is the consensus mainstream position. Contrary to a widespread belief, this book establishes that ontological and epistemological reductionism stand or fall together. The author proposes a new strategy of conservative theory reduction that operates by means of the construction of functional sub-concepts that are coextensional with physical concepts. Thus, a complete conservative reductionism is established that vindicates both the indispensable scientific character of the special sciences and their reducibility to physics. The second part of the book works this strategy out, using the example of classical and molecular genetics.