This thesis investigates the metaphysical nature of necessity. My study focuses primarily on the reduction of metaphysical necessity and the question of whether a necessary truth can be reductively defined. Theodore Sider develops a new reductive account of metaphysical necessity. Unfortunately, the multiple realizability problem posed by Jonathan Schaffer undermines the credibility of Sider’s account. This underlies my motivation to search for a revised Siderian account of necessity. On this basis, I propose a modified version of Sider’s account and argue (...) that analytic, natural-kind and micro-reduction truths are necessary truths if and only if they express the same states of affairs as logical truths. Since logical truths are necessary truths, analytic, natural-kind and micro-reduction truths are necessary truths as well. In this thesis, I will proceed as follows. Chapter 1 introduces Sider’s account of necessity. In particular, I focus on his analysis of the necessity of a micro-reduction truth. His analysis is largely constitutive of the notion of a metaphysical semantics and some associated notions. I reconstruct his analysis and articulate his metaphysical semantics and the associated notions. Chapter 2 presents and clarifies the multiple realizability problem of Schaffer. His critique shows that Sider’s metaphysical semantics cannot handle multiple realizability. Sider refutes this claim by arguing that the multiple realizability problem is a problem for his analysis of the necessity of a micro-reduction truth, not for his metaphysical semantics itself. This is a good starting point to propound a modified account of necessity. Chapter 3 proposes and articulates my modified analysis of the necessity of a micro-reduction, where I show that the necessity of a micro-reduction truth can be reductively defined without appeal to Sider’s metaphysical semantics. I argue that analytic, natural-kind and microreduction truths are necessary truths if and only if they express the same states of affairs as logical truths. Logical truths are necessary and so are they. After that, I consider a potential objection to my modified account and attempt to respond to it. (shrink)