"When they became unwilling, like DESCARTES and STAHL, to have constant recourse in their explanations, to the soul, they tried to find a philosophic proof of the existence of material forces, to show that matter, as mere matter, is endowed with particular forces, with which they might satisfactorily explain a great many of its phenomena. No one had hitherto sought for a similar proof; for ARISTOTLE had contented himself with an axiom, that all natural things contain in themselves the sufficient cause of their movement and rest. GLISSON and LEIBNITZ set themselves in search of this proof; but it was reserved for the immortal KANT to find it in the nature of matter itself. "FRANCIS GLISSON may with propriety be considered as the precursor of LEIBNITZ. What he tried to demonstrate by scholastic subtlety, and by thousands of syllogisms, was developed by LEIBNITZ with a clearness and ability, which secured the suffrages, even of the unenlightened. Both of them went too far, in attributing life and sensation to matter, instead of claiming for it the two simple and primordial forces of attraction and repulsion. "GLISSON sets out with the idea of substance, but he does not explain it with sufficient precision. Every substance has three substantial rudiments,--_fundamental_ substance, by means of which it exists,--_energetic_ substance, by means of which it acts,--and _additional_ substance, which determines its accidental qualities. All matter, as substance, must have an energetic substance or nature, which is the internal principle of movement. Therefore whatever moves spontaneously, and in virtue of an internal force, must _feel_ this motion, _and desire it_. All matter feels that it is, and that it exists by itself. It has therefore, consciousness of its own nature. Life consists in the activity of the internal substantial energetic nature. Death is the dissolution of the triple alliance of the internal energetic substantial nature, with the vegetative and animal natures, which two last belong to the _additional_ substance."[28] In applying his theory to physiology, GLISSON'S idea is, that the fibres of the human body are endowed with a force, which he divides into three kinds; to wit, natural or inherent force, (robur insitum)--vital force, (robur vitale)--and animal force, (robur animale.) Natural or inherent force, is a part of the constitution of the fibre, and is as much a property of its organization as are its tenacity, tensibility, &c. The sum of this force varies, in proportion as the constitution of the fibre is more or less perfect. It is strongest in athletic men and strong animals, and weaker in relaxed and debilitated persons. It may be compared with the contractilite de tissu of BICHAT.