the demon scrupulously takes part in all these matters, sees all things, understands all things, and dwells in the most profound recesses of the mind, in the place of conscience. He of whom I speak is entirely our guardian, our individual keeper, our watcher at home, our own proper regulator, a searcher Into our inmost fibres, our constant observer, our inseparable witness, a reprover of our evil actions, an approver of our good ones; if he is becomingly attended to, sedulously examined and devoutly worshipped, in the way in which he was worshipped by Socrates in justice and in innocence; he is our forewarner in uncertainty, our monitor in matters of doubt, our defender in danger, and our assistant in need. He is able also by dreams, and by tokens, and perhaps even openly, when necessity demands it, to avert from you evil, to increase your blessings, to aid you when depressed, to support you when falling, to lighten your darkness, to regulate your prosperity, and modify your adversity.
This part is quoted in a talk on the Thelemic concept of Holy Guardian Angel here.