103. Taking as our ‘system of bodies’ the whole physical universe, we now see that, according to the test we have already laid down, energy has as much claim to be regarded as an objective reality as matter itself. But the forms of statement are most markedly different for the two. We before spoke of the quantity of matter without qualification, but we now speak of _the sum of the two kinds_ of energy. Let us think for a moment of this, and we see that whereas (to our present knowledge, at least) matter is always the same, though it may be masked in various combinations, energy is constantly changing the form in which it presents itself. The one is like the eternal, unchangeable Fate or _Necessitas_ of the antients; the other is Proteus himself in the variety and rapidity of its transformations. Φύσις, διαδόχαις σχημάτων τρισμυρίοις ἀλλάσσεται τύπωμα, Πρωτέως δίκην, πάντων ὅσ’ ἔστι ποικιλώτατον τέρας· τῆς δ’ αὖτ’ Ἀνάγκης ἐστ’ ἀκίνητον σθένος, μόνη δ’ ἁπάντων ταὐτὸ διαμένουσ’ ἀεὶ βροτῶν τε καὶ θεῶν πάντ’ ἀποτρύει γένη.[37] 104. And again, _energy is of use to us solely because it is constantly being transformed_. When the sluice is shut, or the fire put out, the machinery stops; when a man cannot digest his food, he breaks down altogether. Coal in itself, except on account of an occasional fossil it may contain, or its still somewhat uncertain mode of formation, or (to take a lower point of view) as a material for ornament, is a very useless thing indeed: its grand value consists in its chemical affinity, in virtue of which it possesses great potential energy as regards the oxygen of the air, which can very easily be transformed into its equivalent in heat. ‘Keep your powder dry’ is merely one way of saying ‘preserve the ready transformability of your energy.’ In fact, if we think for a moment over what has just been said, to the effect that the only real things in the physical universe are matter and energy, and that of these matter is simply passive, it is obvious that all the physical changes which take place, including those which are inseparably associated with the thoughts as well as the actions of living beings, are merely transformations of energy. Thus it is an inquiry of the very utmost importance as regards the present universe: _Are all forms of energy equally susceptible of transformation?_ To see the importance of this question, the reader has only to reflect that if there be any one form of energy less readily or less completely transformable than the others, and if transformations constantly go on, more and more of the whole energy of the universe will inevitably sink into this lower grade as time advances. Hence the whole possibility of transformation must steadily grow less and less; in scientific language, though the _quantity_ of energy remains for ever unchanged, its _availability_ steadily decreases.