The subject of this thesis is the relationship of the psychology and epistemology of the philosopher Epictetus (c.55-135 A.D.) to the Stoic tradition to which he belonged. Epictetus' psychology is characterised by the subsuming of psychological functions and development under the single term prohairesis, and his epistemology under a corresponding term 'use of impressions' (chrêsis phantasiôn); the two terms being used largely interchangeably. The status of the prohairesis as a faculty is investigated and it is concluded that it is the principal over-arching faculty of the governing-principle of the soul (hêgemonikon), and thus occupies a similar position to that occupied by Reason (logos) in the Stoic tradition. Prohairesis is used to define the 'end' and 'good' for man in Epictetus' teaching, displacing 'wisdom' and 'virtue' with 'prohairesis as it ought to be'. Epictetus thus seeks to reflect the concerns of the 'person making progress' by making the end an extreme point on a continuum of mental and moral states on which he already occupies a place.

The self-contemplating ability of prohairesis allows it to be used to define the human self as a single unified consciousness, and also distinguish it from 'external' things outwith our power. The redefining of all things external to the mind as outwith our power is Epictetus' most significant innovation.

Epistemology is closely linked to psychology. The 'use of impressions' involves using 'preconceptions' (prolêpseis) and 'rules' (kanones) to test incoming impressions. The aim is not to allow any false judgement to be installed permanently in the prohairesis and thus hinder moral progress. The substitution of the term judgement (dogma) for 'cognition' (katalêpsis) allows Epictetus to draw attention to the need to replace poor judgements with better ones.

The thesis concludes that Epictetus introduces a number of innovations to Stoicism and also develops established doctrines to reflect his emphasis on the 'inner life' and on the moral development of ordinary people.