"Heed these words, you who wish to probe the depths of Nature: If you do not find within your Self that which you seek, neither will you find it outside. If you ignore the wonders of your own House, how do you expect to find other wonders? In you is hidden the Treasure of Treasures. Know Thyself and you will know the Universe and the Gods." - Temple of Apolla at Delphi, by Proxy of Luxor, Ancient Egypt
Self Mastery is the process by which a person comes to truly "Know Thyself". So much of our lives and who we identify ourselves as, is the result of how we've been told to think, to act, to feel, and how we have learned through our lives to cope with the reality we're faced with. In essence, we have replaced our ignorance of the universe and ourselves with advice based on "getting by". Self Mastery is the process of peering underneath the covers to find those answers out for ourselves, and by doing so to truly come to know who and what we are, and what our relationship is with Divinity, the Universe, and the totality of reality. In finding this, in truly getting to Know Ourselves, we achieve a certain harmony with Nature, Divinity, and the Universe which allows us to operate in unison with those things.
Self Mastery is not about brute-forcing Reality into submission, it's about reaching a deep experiential understanding and harmony with how ourselves, and reality, operates. To do this we have to first become open to experience, first hand, all of that previous programming and assumptions we've accumulated, all of those aspects of our lives and reality which give us cause for concern. When that is accomplished, when we know through direct experience who and what we are in this grand system we call Reality, we come to experience a harmony within ourselves whose effects are far beyond what we might imagine.
There is a means and mechanism behind the workings of Reality. Learning and experiencing precisely how that works leads to a mode of operation which places that constant experience and understanding at the center of your daily life. When that happens, there is truly nothing which is concealed from your understanding which cannot be known, directly, by you. When that happens, you will truly realize that Your Mind Has No Boundaries.