This thesis proposes a consideration of Berkeley's thought from the standpoint of its roots, rather than (which is the prevalent perspective in today's Berkeley scholarship) from the point of view of the developments that this thought has brought about in modern philosophy. Chapter One deals with a number of specific introductory issues, and then delineates a theoretical context within which my own approach will reveal its scholarly significance. In Chapter Two I advance the idea that there is in Berkeley's early writings an entire network of Platonic features, attitudes, and mind sets, prefiguring his speculative and openly Platonic writing Siris. Chapter Three is a systematic attempt at considering Berkeley's immaterialist philosophy in close connection to the topic of liber mundi, with the twofold objective of pointing out those of the medieval implications of the topic that Berkeley preserved, and the "novelties" he brought forth in his use of the topic. The central idea around which Chapter Four is clustered is that, in Siris, Berkeley comes to make use of one of the most ancient "spiritual techniques": alchemy. Berkeley’s arguments and notions in Siris will be discussed by constant reference to alchemic notions, writings and authors. Chapter Five is an attempt at considering Berkeley’s thought from the standpoint of the Christian apologetic tradition, and its objective is to show that one of the roots of Berkeley's thought could be found precisely in this tradition. In Chapter Six I will show that even when designing such a practical project as the "Bermuda scheme" Berkeley was under the modeling influence of the past. More precisely, the chapter purports to offer a discussion of Berkeley's "Bermuda scheme" in light of the Western representations of the "happy islands", "earthly paradise", ''eschaton". The last chapter (Chapter Seven) purports to undertake a comparative analysis of some of the ideas professed by medieval Catharism, and George Berkeley’s denial of the existence of matter. The central notion around which my comparative approach is articulated is the idea that, in both cases, matter is regarded as the source of evil. What I will try to show is that Berkeley's attitudes to the material world echoed certain Cathar theological anxieties and patterns of thought.