_The_ Idea of Cause, _modified into the conceptions of mechanical cause, or Force, and resistance to force, or Matter, is the foundation of the Mechanical Sciences; that is, Mechanics,_ (_including Statics and Dynamics,_) _Hydrostatics, and Physical Astronomy._ (III. 1.) XLVI. _The Idea of Cause is not derived from experience; for in judging of occurrences which we contemplate, we consider them as being, universally and necessarily, Causes and Effects, which a finite experience could not authorize us to do. The Axiom, that every event must have a cause, is true independently of experience, and beyond the limits of experience._ (III. 2.) XLVII. _The Idea of Cause is expressed for purposes of science by these three Axioms_:--Every Event must have a Cause:--Causes are measured by their Effects:--Reaction is equal and opposite to Action. (III. 4.) XLVIII. _The Conception of Force involves the Idea of Cause, as applied to the motion and rest of bodies. The conception of_ force _is suggested by muscular action exerted: the conception of_ matter _arises from muscular action resisted. We necessarily ascribe to all bodies solidity and inertia, since we conceive Matter as that which cannot be compressed or moved without resistance._ (III. 5.) XLIX. _Mechanical Science depends on the Conception of Force; and is divided into_ Statics, _the doctrine of Force preventing_ {14} _motion, and_ Dynamics, _the doctrine of Force producing motion._ (III. 6.) L. _The Science of Statics depends upon the Axiom, that Action and Reaction are equal, which in Statics assumes this form_:--When two equal weights are supported on the middle point between them, the pressure on the fulcrum is equal to the sum of the weights. (III. 6.) LI. _The Science of Hydrostatics depends upon the Fundamental Principle that_ fluids press equally in all directions. _This principle necessarily results from the conception of a Fluid, as a body of which the parts are perfectly moveable in all directions. For since the Fluid is a body, it can transmit pressure; and the transmitted pressure is equal to the original pressure, in virtue of the Axiom that Reaction is equal to Action. That the Fundamental Principle is not derived from experience, is plain both from its evidence and from its history._ (III. 6.) LII.