This thesis examines the theme of secrecy and silence in the philosophy of Plotinus. This philosopher is known for the innovative use he made of Platonic and Aristotelean materials in constructing a thought-world which posits a totally transcendent first principle, the one or the good, from which all other entities are derived. The Plotinian one is ineffable by its very nature, and Plotinus expounds at length the claim that it cannot be comprehended, either by speech or thought. The paradox of a philosopher writing extensive discourses about a reality which is said to be ineffable is the main topic of this thesis, which seeks to answer the question: what is Plotinus doing when he tells us that he cannot, or will not, reveal the nature of the one? Partial answers to this question have been given in previous scholarship through study of the philosophic background which led Plotinus to posit such an ineffable reality, and through analysis of the arguments in which he upholds the doctrine of the one’s ineffability. Building on this body of work, this thesis gives a more compre- hensive answer to this question by analysing the tropes of silence and secrecy which were developed in Middle Platonism, derived especially from Plato’s writings, and by locating Plotinus in a broader philosophic tradition which interpreted canonical thinkers as esoteric writers. In this way, the thesis provides a historical context for Plotinus’ treatment of the ineffable one. Plotinus’ discourse of ineffability is present- ed not just as a response to purely philosophical issues, but also an enactment of a tradition of philosophic silence, which determined in part how a Platonist philoso- pher articulated in written form ideas about concealment and the limits of discourse. Through a combination of close reading of a number of Plotinian texts and full dis- cussion of the wider context, this thesis aims to integrate analytical and cultural approaches to Plotinus’ philosophy. It aims also to bring out the significance of the theme of philosophical silence for late antique philosophy both as a discipline and as a socially-embedded part of Græco-Roman civilisation.