Page:The System of Nature.djvu/151

 and visible signs. The brain, affected by a modification which is called FEAR, diffuses a paleness over the countenance, excites a tremulous motion in the limbs called trembling. The brain, affected by a sensation of GRIEF, causes tears to flow from the eyes, even without being moved by any exterior object; an idea which it retraces with great strength, suffices to give it very little modifications, which visibly have an influence on the whole frame.

In all this, nothing more is to be perceived than the same substance which acts diversely on the various parts of the body. If it be objected that this mechanism does not sufficiently explain the principles of the motion or the faculties of the soul; we reply, that it is in the same situation as all the other bodies of Nature, in which the most simple motion, the most ordinary phenomena, the most common modes of action are inexplicable mysteries, of which we shall never be able to fathom the first principles. Indeed, how can we flatter ourselves we shall ever be enabled to compass the true principle of that gravity by which a stone falls? Are we acquainted with the mechanism which produces attraction in some substances, repulsion in others? Are we in a condition to explain the communication of motion from one body to another? But it may be fairly asked,&mdash;Are the difficulties that occur, when attempting to explain the manner in which the soul acts, removed by making it a spiritual being, a substance of which we have not, nor cannot form one idea, which consequently must bewilder all the notions we are capable of forming to ourselves of this being? Let us then be contented to know that the soul moves itself, modifies itself, in consequence of material causes, which act