The idea of the Will or the True Will was first explored in writing by Eliphas Levi and further explored and publicized by Aleister Crowley. It has also been identified as the True Self, but this term is somewhat limiting. The Will is defined roughly as your true path in alignment with your purpose in the Universe. You are your Will and everything else, your petty desires, disappointments, lusts, are simply built up around this Will, they are window dressing. The Will isn’t the soul though. It is something more akin to your own personal position within the functioning Universe.
According to Thelema doctrine, actions that are in accordance with your true will are correct actions, and those opposed to it are incorrect, regardless of the mundane morality attached to them. Thus, “Do as thou Wilt shall be the whole of the law”. While it may seem that this encourages selfish action even at the expense of others, Thelema holds that observing and acting in accordance to your True Will won’t have any negative effect on others who are also acting in accordance to their own True Will.
“Thou hast no right but to do thy will. Do that and no other shall say nay. For pure will, unassuaged of purpose, delivered from the lust of result, is every way perfect.”
Take this carefully; it seems to imply a theory that if every man and every woman did his and her will—the true will—there would be no clashing. “Every man and every woman is a star,” and each star moves in an appointed path without interference. There is plenty of room for all; it is only disorder that creates confusion.
From these considerations it should be clear that “Do what thou wilt” does not mean “Do what you like.” It is the apotheosis of Freedom; but it is also the strictest possible bond.
Do what thou wilt—then do nothing else. Let nothing deflect thee from that austere and holy task. Liberty is absolute to do thy will; but seek to do any other thing whatever, and instantly obstacles must arise. Every act that is not in definite course of that one orbit is erratic, an hindrance. Will must not be two, but one.1
Other traditions take a softer view of the Will, allowing more wiggle room with regard to accomplishing it within the context of our own communities, but these also do not tend to give such detailed definitions of the True Will.