If we wish to understand the origin and aims of alchemy we must endeavour to recreate the atmosphere of the Middle Ages, and to look at the subject from the point of view of the alchemists themselves. Now, this atmosphere was, as I have indicated in a previous essay, surcharged with mystical theology and mystical philosophy. Alchemy, so to speak, was generated and throve in a dim religious light. We cannot open a book by any one of the better sort of alchemists without noticing how closely their theology and their chemistry are interwoven, and what a remarkably religious view they take of their subject. Thus one alchemist writes: "In the first place, let every devout and God-fearing chemist and student of this Art consider that this arcanum should be regarded, not only as a truly great, but as a most holy Art (seeing that it typifies and shadows out the highest heavenly good). Therefore, if any man desire to reach this great and unspeakable Mystery, he must remember that it is obtained not by the might of man, but by the grace of God, and that not our will or desire, but only the mercy of the Most High, can bestow it upon us. For this reason you must first of all cleanse your heart, lift it up to Him alone, and ask of Him this gift in true, earnest and undoubting prayer. He alone can give and bestow it."(1) Whilst another alchemist declares: "I am firmly persuaded that any unbeliever who got truly to know this Art, would straightway confess the truth of our Blessed Religion, and believe in the Trinity and in our Lord JESUS CHRIST."(2) (1) _The Sophic Hydrolith; or, Water Stone of the Wise_. (See _The Hermetic Museum_, vol. i. pp. 74 and 75.) (2) PETER BONUS: _The New Pearl of Great Price_ (trans. by A. E. WAITE, 1894), p. 275. Now, what I suggest is that the alchemists constructed their chemical theories for the main part by means of _a priori_ reasoning, and that the premises from which they started were (i.) the truth of mystical theology, especially the doctrine of the soul's regeneration, and (ii.) the truth of mystical philosophy, which asserts that the objects of Nature are symbols of spiritual verities. There is, I think, abundant evidence to show that alchemy was a more or less deliberate attempt to apply, according to the principles of analogy, the doctrines of religious mysticism to chemical and physical phenomena. Some of this evidence I shall attempt to put forward in this essay. In the first place, however, I propose to say a few words more in description of the theological and philosophical doctrines which so greatly influenced the alchemists, and which, I believe, they borrowed for their attempted explanations of chemical and physical phenomena. This system of doctrine I have termed "mysticism"--a word which is unfortunately equivocal, and has been used to denote various systems of religious and philosophical thought, from the noblest to the most degraded. I have, therefore, further to define my usage of the term.