2. _Matter._--The conception of Force, as we have said, arises with our consciousness of our own muscular exertions. But we cannot imagine such exertions without also imagining some bodily substance against which they are exercised. If we press, we press something: if we thrust or throw, there must be something {207} to resist the thrust or to receive the impulse. Without body, muscular force cannot be exerted, and force in general is not conceivable. Thus Force cannot exist without _Body_ on which it acts. The two conceptions, Force and Matter, are co-existent and correlative. Force implies resistance; and the force is effective only when the resistance is called into play. If we grasp a stone, we have no hold of it till the closing of the hand is resisted by the solid texture of the stone. If we push open a gate, we must surmount the opposition which it exerts while turning on its hinges. However slight the resistance be, there must be some resistance, or there would be no force. If we imagine a state of things in which objects do not resist our touch, they must also cease to be influenced by our strength. Such a state of things we sometimes imagine in our dreams; and such are the poetical pictures of the regions inhabited by disembodied spirits. In these, the figures which appear are conspicuous to the eye, but impalpable like shadow or smoke; and as they do not resist the corporeal impressions, so neither do they obey them. The spectator tries in vain to strike or to grasp them. Et ni docta comes tenues sine corpore vitas Admoneat volitare cavâ sub imagine formæ, Irruat ac frustra ferro diverberet umbras. The Sibyl warns him that there round him fly Bodiless things, but substance to the eye; Else had he pierced those shapes with life-like face, And smitten, fierce, the unresisting space. Neque illum Prensantem nequicquam umbras et multa volentem Dicere, preterea vidit. He grasps her form, and clutches but the shade. Such may be the circumstances of the unreal world of dreams, or of poetical fancies approaching to dreams: for in these worlds our imaginary perceptions are bound by no rigid conditions of force and reaction. In {208} such cases, the mind casts off the empire of the idea of cause, as it casts off even the still more familiar sway of the ideas of space and time. But the character of the material world in which we live when awake is, that we have at every instant and at every place, force operating on matter and matter resisting force.