This, if I understand it, is one half the new theory; the other half is not less acceptable. Newton had described motion as a result of two principles: the first, inertia, was supposed to be inherent in bodies; the second, gravity, was incidental to their co-existence. Yet inherent inertia can only be observed relatively: it makes no difference to me whether I am said to be moving at a great speed or absolutely at rest, if I am not jolted or breathless, and if my felt environment does not change. Inertia, or weight, in so far as it denotes something intrinsic, seems to be but another name for substance or the principle of existence: in so far as it denotes the first law of motion, it seems to be relative to an environment. It would therefore be preferable to combine inertia and attraction in a single formula, expressing the behaviour of bodies towards one another in all their conjunctions, without introducing any inherent forces or absolute measures. This seems to have been done by Einstein, or at least impressively suggested: and it has been found that the new calculations correspond to certain delicate observations more accurately than the old. This revolution in science seems, then, to be perfectly legal, and ought to be welcomed; yet only under one important moral condition, and with a paradoxical result. The moral condition is that the pride of science should turn into humility, that it should no longer imagine that it is laying bare the intrinsic nature of things. And the paradoxical result is this: that the forms of science are optional, like various languages or methods of notation. One may be more convenient or subtle than another, according to the place, senses, interests, and scope of the explorer; a reform in science may render the old theories antiquated, like the habit of wearing togas, or of going naked; but it cannot render them false, or itself true. Science, when it is more than the gossip of adventure or of experiment, yields practical assurances couched in symbolic terms, but no ultimate insight: so that the intellectual vacancy of the expert, which I was deriding, is a sort of warrant of his solidity. It is rather when the expert prophesies, when he propounds a new philosophy founded on his latest experiments, that we may justly smile at his system, and wait for the next.