The ancient Greeks, when they looked about them on the world in which they lived, came to the definite conclusion that everything is in a constant state of flux, or change. Things animate and inanimate gradually disintegrated and tended either to disappear (apparently) or to change into other forms of matter. With their true æsthetic sense, they felt it necessary that there should be some _one_ permanent thing in the world, underlying all the changes which they saw going on about them, and many of their early speculations were devoted to the nature and constitution of this one “permanent thing.” Thales, of Myletus, who flourished about 585 B. C., and who was, perhaps, the first great philosopher and physicist, contended that the essential principle of things,--the substance, or stuff, of all things,--must be _water_. He held the view that, by condensation and rarefaction of water all things rise, and he actually attempted an evolutionary account of the Genesis of Man, Plants and Animals, with this idea as a basis for his thought. Anaximenes said that _air_, or _ether_, must be the substance of things. Heraclitus regarded _fire_ as the most primary element in the universe,--from which all else arises. Anaximander said that the “unlimited”--a sort of boundless, animated mass--is the ultimate substance. Plato, as we know, contended that the permanent reality of things was not anything material at all, but was mind, or _spirit_. Empedocles, (495–435 B. C.) advanced the theory that there are four elements--Earth, Air, Fire and Water. Anaxagoras contended that nothing changed of itself, but that it is caused or made to change, and that _that_ which produces these changes is the permanent reality. This he believed to be a sort of mind or universal intelligence (_Nous_), but he regarded this mind as strictly impersonal, as well as immaterial, and did not attempt to answer the difficulty as to how mind can affect matter in any detailed manner. It was only natural that, prior to the discovery of the laws of the indestructibility of matter and energy, that this sense of “change” should have struck these early thinkers very forcibly, since they had no means of ascertaining that, when matter disappears from our sight, it is not actually destroyed. We now know that, when we burn a candle, the candle disappears, but that the elements composing the candle are merely changed into invisible gaseous compounds, which are no longer visible to the human eye. Lacking delicate instruments of precision, the ancients could not know this; to them, the matter of the candle would have disappeared. Hence, it was only natural that they should seek the ultimate reality behind these changes, and speculate as to its origin and nature. ALCHEMY