M.A. / The function of the Principles in the system rendered in W. B. Yeats’s A Vision (1937), like most aspects of the system, has received minimal critical and scholarly attention. The reason for this state of affairs is that most Yeats scholars prefer to avoid studying A Vision, for various reasons. The result of this is that little is known of the system. Certain scholars have argued that A Vision is a hoax and an incomprehensible work, which need not be elucidated. The spiritual origin and nature of A Vision are discouraging to critics and scholars, who prefer to interpret the poems that A Vision influenced without referring to the system. This dissertation viewed A Vision as a significant work within the general category of Yeats studies. The system of A Vision is not only useful to interpreting many poems, but is an extraordinary body of work that can be studied in isolation. There is a serious need to elucidate the system, in order to yield more knowledge on this significant aspect of the Yeats corpus. This research aimed at elucidating an important aspect of the system, the function of the Principles. A central premise of the system is that human beings can be classified according to twenty-eight typical incarnations. Every being is set to embody each incarnation. Reincarnation is then central to the system’s account of existence. The primary function of the Principles is to enable an individual being to incarnate, and to pass from one incarnation to the next. Essentially, the Principles are imperative to the system’s exposition on the cycle of reincarnation. To inform its exposition of the Principles, this study delved into the vast body of work produced by the psychical communications, which elucidated the system. These communications are referred to as the Automatic Script. George Mills Harper transcribed the Automatic Script, which was published in the four volumes of Yeats’s Vision Papers (1992). In essence, the Automatic Script is the spiritual and mystical origin of the system rendered in A Vision. For this reason, Yeats’s Vision Papers are crucial to a comprehensive exposition of the system. In addition, this study referred to many of Yeats’s essays on topics ranging from the “ultimate reality” to the Anima Mundi, from the process of symbolisation to the Daimon. Furthermore, various poems were employed to illustrate the importance of the system in interpreting Yeats’s poems. This study defined the state of existence in the transcendent realm, which is imperative to elucidating the function of the Principles in the system. The “ultimate reality” is the source of all incarnate spirits, for this reason it was necessary to describe what is known of this reality. Then the details of the process of incarnation were clarified. The Principles were defined and their functions in the process of incarnation were illustrated. In addition, this study discovered that the Principles do have an effect upon the incarnate being, even though they are passive in life. The function of the Principles during incarnation was elucidated in terms of relation between the Principles and the material Faculties, which are the mediators of the incarnate experience. Scholars who have read A Vision have met the geometry of the system with much aversion. The geometry has been described as flawed and inconsistent. Certain scholars have gone so far as to argue that A Vision makes more sense when the geometry is ignored. This dissertation illustrated that the geometry is imperative to elucidating the system. The geometry is not only necessary to clarifying the system, but aids any attempt to comprehend many aspects of A Vision. Lastly, this study illustrated the function of the Principles in death. The disincarnate states of the soul were explored to illustrate that the Principles define the states between incarnations. Through clarifying the process of death it is clear that, according to the system, existence is characterised by a perpetual return to its point of origin.