Naturally, strategy is not an exact science like mathematics, physics, or engineering--at least not now. Whether it ever will be cannot be foretold. The reason that strategy (like medicine and most other sciences concerning human beings) is not an exact science is simply because it involves too many unknown quantities--quantities of which our knowledge is too vague to permit of our applying exact methods to them, in the way in which we apply exact methods to the comparatively well-known quantities and elements in the so-called "exact sciences." But a science may be a science even if it is not an exact science; we may know certain important principles sufficiently well to use them scientifically, even if we do not know them with sufficient exactness to permit us to use them as confidently as we should like. We may know, for instance, that it is folly to divide a military force in the presence of an active enemy into such small forces, and at such distances apart, as to let the enemy defeat each small force, one after the other, even if we do not know exactly how far it would be safe to separate two forces of a given size, in the presence of an enemy of a given power. It is well to know a fact in general terms, even if we do not know it in precise terms: it is well to know in general terms that we must not take prussic acid, even if we do not know exactly how much is needed to kill. So the studies and problems instituted by Von Moltke, and copied in all the armies and navies of the world, have brought about a science of strategy which is real, even though not exact, and which dwells in the mind of each trained strategist, as the high tribunal to which all his questions are referred and by whose decisions he is guided; just as the principles of medicine are the guide alike of the humblest and the most illustrious practitioner, wherever the beneficent art of medicine is practised.