The flame of an ordinary fire can still be a thing of wonder to the man whose mind is open to receive impressions even from the commonplace. How illusive it is!--dancing, darting, flickering, flashing--appearing, disappearing--unsubstantial yet active and almost miraculously potent. The effect upon the mind of primitive man must have been keen and vivid to the highest degree, and must have produced results of corresponding significance upon his spiritual development. But the deeper kind of wonder is reserved for the systematic speculative thinker, whose attention is arrested by the phenomena of a steadily burning flame, say that of a lamp. The oil is sucked up into the wick and slowly decreases in volume. At the point where the flame begins it rises in vapour, becomes brilliant, and, in the case of a clear flame, disappears. There is thus a constant movement from below upwards. The flame has all the appearance of a "thing," with comparatively definite form and continued existence, and yet is never really the same, not for the minutest fraction of a moment. It is an appearance born of incessant motion--let the motion stop, the flame is gone. Where the burning is accompanied by smoke, there is an apparent return of volatilised matter to solid form. Now let a philosopher like Heracleitus be meditating on nature as a circulatory system, and let him, by chance or otherwise, bring together in his mind the phenomena of a burning lamp and the cosmic facts for which he seeks an explanation--is it difficult to imagine his Eureka? At any rate, Heracleitus felt that in the phenomena of combustion he had gained an insight into the ultimate constitution of nature. And he concluded from them that there is no such thing as substance, properly so called, but simply constant movement; the movement _is_ substance. The great solid-seeming cosmos is motion; some of it visible, some of it imperceptible; some of it rising upward to serve as fuel, some of it falling downward, after having fed the flame, to form the constituents of the present world. The motion is constant, the stream ever-flowing: no "thing" is ever at rest, and, if it were at rest, would disappear.