Anselm is the outstanding Christian Latin philosopher and theologian of the eleventh century. However, Anselm’s philosophical thought has rarely been seen as a whole. The interrelation between Anselm’s core philosophical theories and their coherence has yet to receive proper attention. My thesis aims to fill this gap. This thesis provides a rational reconstruction of Anselm’s thought on semantics, modality and free will. I not only bring to light the originality of Anselm’s individual philosophical theories but also show how Anselm’s core philosophical theories mutually support each other, and how his use of certain concepts in many places gives it a coherence that has not often been noticed. Moreover, Anselm’s theories of semantics, modality, and ethics build up a unified, theistic metaphysics. Instead of fitting Anselm into the Platonic, Neoplatonic, Augustinian, or Scholastic traditions, I emphasise that Anselm develops a coherent and unique realism of his own. Generally speaking, his realism is power-based and teleological. Everything, directly or indirectly, is assigned a set of powers – natural power, rational power, the power of free will, the power to signify, and so forth – and every power achieves a divine purpose. This power-based and teleological metaphysical worldview is twofold – natural and ethical. On the one hand, God creates every individual being and regulates their purpose. A creature attains its purpose (i.e., in the position that it ought to be) and participates in God’s rectitude. On the other hand, rational creatures are given a privileged power – free will – of which the purpose is to help them acquire justice and in pursue what they ought to will. Yet it also entitles rational agents to deviate from what they ought to do. The structure of free will constitutes the source of genuine contingency against the background of theism.