The Philosophy of Natural Magic, by Henry Cornelius Agrippa, L. W. de Laurence ed. , at sacred-texts.com
The philosophers, especially the Arabians, say that man's mind, when it is most intent upon any work, through its passion and effects, is joined with the mind of the stars and intelligences; and, being so joined, is the cause of some wonderful virtue being infused into our works and things; and this, because there is in the mind an apprehension and power of all things, so all things have a natural obedience to it, and of necessity an efficacy; and more to that which desires them with a strong desire. And according to this is verified the art of characters, images, enchantments, and some speeches, and many other wonderful experiments as to everything which the mind affects. By this means, whatsoever the mind of him that is in vehement love, affects, hath an efficacy to cause love; and whatsoever the mind of him that strongly hates, dictates, hath an efficacy to hurt and destroy. The like is in other things, which the mind affects with a strong desire. For all those things which the mind acts and dictates by characters, figures, words, speeches, gestures, and the like, help the appetite of the soul and acquire certain wonderful virtues; as from the soul of the operator, in that hour when such a like appetite doth invade it, so from the opportunity and celestial influence, moving the mind in that manner. For our mind, when it is carried upon the great excess of any passion or virtue, oftentimes presently takes of itself a strong, better and more convenient hour or opportunity, which Thomas Aquinas, in his third
book against the Gentiles, confesseth. So many wonderful virtues both cause and follow certain admirable operations by great affections in those things which the soul doth dictate in that hour to them. But know that such things confer nothing, or very little, to the author of them, and to him which is inclined to them, as if he were the author of them. And this is the manner by which their efficacy is found out. And it is a general rule in them, that every mind that is more excellent in its love and affection makes such like things more fit for itself, becoming efficacious to that which it desires. Every one, therefore, that is willing to work in Magic must know the virtue, measure, order, and degree of his own soul, in relation to the Power of the Universe.