Epistemological: PB's presentation of epistemology in the light of mentalism is a unique and extraordinary tool for those who seek to understand the ‘whys’ as well as the ‘hows’ of spiritual technique.  Mentalism gives us a clear understanding of the mind and a revolutionary reorientation of our awareness of its relationship to the body (which is inside it!) and the world (also inside mind!)  Mentalism is a non-doctrinal, unvarnished examination of the way the mind works; it welcomes the observations of modern science, and incorporates their truths into the greater truth of consciousness itself.  Armed with an understanding of mentalism we can readily evaluate the viability of any meditation technique and easily recognize, for ourselves, the fundamental factors of philosophy that must be encountered and explored by every tradition. 

Epistemology is the gateway to Philosophy—or rather the co-gateway together with mystical experience.  Epistemology asks us to ask the question: “How do we know what we know, and how (if at all) can we know a truth?”  Any consideration of the validity of our individual or common experience, of the relevance of transcendent states and metaphysical doctrine, and the means to differentiate delusion, illusion, and reality begins with Epistemology.  Without mastering it, our minds remain vulnerable to subjective emotions and unconscious beliefs; with mastery comes detachment and objectivity, and the courage to enquire into the unwelcoming depths of the Unknown.

While there are many plausible epistemological schemes, mentalism is by far the most accurate and immediate.  After all, we cannot really jump outside our own minds no matter what we’d like to believe or logically construe—so why not begin where we are and see where it takes us?  Of course PB’s approach to mentalism is much more thoroughgoing and well-reasoned than this brief blurb could ever be—in fact, one of the points he makes is that mentalism is not merely a memorized conclusion (like “all is mind”) but rather it is an ongoing understanding of our immediate experience and our dynamic reasoning as experience.  This understanding can only be gained by approaching these texts with energy, patience, and the willingness to go over them again and again, until they read as easily as a novel, and the reasoning embedded in them transforms one’s own perception of the world.  After all, this phase comes after meditation, and is meaningless—dangerous even—if it is taken to be a theoretical discipline.  It is not.  It is the transformation of our subjective awareness from the feeling-based experiences of mysticism into the knowing-based perspective of philosophy.

Books to read:

The Hidden Teaching Beyond Yoga - The doctrine of mind as the basis of experience is called Mentalism by PB, and this text is a painstaking guide to understanding it—more, to experiencing its truth for oneself.  This book is meant to be studied, not merely read, and requires the very thing it teaches—a disciplined mind—to do so.  There are many “reasons” to put off reading this book, but it (like all of its ilk) represents a true guardian of the threshold beyond which lies spiritual freedom—and, ironically, the ability to use reason within the greater context of mystical philosophy.

The Wisdom of the Overself - This book is more than a synthesis of the Ancient East and Modern West; it is a new alloy of these perspectives created with the catalyst of PB’s own Insight.  Where The Hidden Teaching Beyond Yoga teaches us to examine and recognize the full potential of our own mind, The Wisdom of the Overself teaches us to look at the cosmos around us in the same light—as World Mind, Itself the active phase of Mind Absolute.  Without an understanding of these principles, the framework of individual mentalism remains incomplete and inevitably vulnerable to the problem of solipsism.  With this additional content, mentalism becomes the means through which we can enter the next phase—the phase of philosophy.
The Notebooks of Paul Brunton:
7. The Intellect (Volume 5) - This Category should be self-evident.  Here PB looks at both sides of the Intellect—its strengths and its weaknesses as well.  He offers us his perspective on many well-known (and lesser-known) thinkers and suggests ways in which the intellect can and cannot be part of the spiritual journey, especially when it comes to the task of enquiring into the nature of Reality.
8. The Ego (Volume 6) - Just because this category was part of the Mystical phase doesn’t mean we’re done with it.  So long as we’re unenlightened, so long must we be aware of the role of the ego in each successive phase of the practice.  We need to learn the wiles and ways of the emotionless subtly arrogant intellectual ego if we are to break the bonds of ignorance and find our way to the Truth.
19. The Reign of Relativity (Volume 13) - The main emphasis in this category is on the deep relativity of the double standpoint—that of the immediate, embodied consciousness in concert with the ultimate and impersonal consciousness.  PB applies the methodology of Western science to Hinduism’s research into the four states of consciousness (waking, subtle, causal, and absolute) and into the Buddhist doctrine of the Void and, as a result, shows us a way to integrate the truths of these traditions into our own understanding, free of any particular dogma.  This is not to say that PB eschews the role of faith in our spiritual journey—but here he is solely addressing those facts of our existence which are independent of such faith—and the understanding of which will go a long way towards stabilizing our meditation experience.
21. Mentalism (Volume 13) - Not one to stand still, PB continued to refine and reflect upon the epistemological approach of mentalism long after he published The Hidden Teaching Beyond Yoga in 1941.  This category contains forty years of his notes on the topic, which incorporates the advances made in theoretical physics, and access to newly discovered or translated ancient texts.  One has to continually consider these paras in terms of our experience of the world, not our conception of the world.  After all, PB is not presenting his theories—he’s describing his own immediate, mindful, engagement with the world.
22. Inspiration and the Overself (Volume 14) - This category is rather more mystical than epistemological, but without it, the study of epistemology is more than likely to go unfinished, for it is an arduous and dry undertaking, and it is in truth the Grace of the Overself that often carries us to the finish line.  Indeed, it is the very presence of the Overself that nudges the mind out of its habitual groove and forces it to look at itself.  Without the inner standpoint of a Glimpse we can never free ourselves from the solipsism of the ordinary mind; without a full integration of mentalism, we will never understand (and therefore never assimilate) a Glimpse.  For it is mentalism that teaches us why these glimpses of the Overself occur, and why they must depart, and it is these very glimpses that transform a study of mentalism into a living experience of the mind.  When these two events have occurred—a Glimpse, and realization of Mentalism, then, and then alone is one genuinely qualified to immerse oneself in the Philosophic Work.