Guidance of _matter_ can be affected by a passive exertion of force without doing work; as a quiescent rail can guide a train to its destination, provided an active engine propels it. But the analogy of the rail must not be pressed: the rail "guides" by exerting force perpendicular to the direction of motion, it does no work but it sustains an equal opposite reaction.[5] The guidance exercised by life or mind is managed in an unknown but certainly different fashion: "determination" can sustain no reaction—if it could it would be a straightforward mechanical agent—but it can utilise the mechanical properties both of rail and of engine; it arranged for the rail to be placed in position so that the lateral force thereby exerted should guide all future trains to a desired destination, and it further took steps to design and compose locomotives of sufficient power, and to start them at a prearranged time. It "employs" mechanical stress, as a capitalist employs a labourer, not doing anything itself, but directing the operations. It is impossible to explain all this fully by the laws of mechanics alone, that is to say, no mechanical analysis can be complete and all-embracing, though the whole procedure is fully subject to those laws. [Footnote 5: It is well to bear in mind the distinction between "force" and "energy." These terms have been so popularly confused that it may be difficult always to discriminate them, but in Physics they are absolutely discriminated. We have a direct sense of "force," in our muscles, whether they be moving or at rest. A force in motion is a "power," it "does work" and transfers energy from one body to another, which is commonly though incorrectly spoken of as "generating" energy. But a force at rest—a mere statical stress, like that exerted by a pillar or a watershed—does no work, and "generates" or transfers no energy; yet the one sustains a roof which would otherwise fall, thereby screening a portion of ground from vegetation; while the other deflects a rain-drop into the Danube or the Rhine. This latter is the kind of force which constrains a stone to revolve in a circle instead of a straight line; a force like that of a groove or slot or channel or "guide."]