

Preliminary Remarks. 7 



bringing together things that are analogous, and making the 

 motion of animated beings to depend upon a cause foreign 

 to matter ; matter being found inert under all other circum- 

 stances in which we have been able to examine it. Another rea- 

 son is given in the schools of philosophy for attributing sponta- 

 neous motion to an immaterial principle ; namely, that the will, 

 by the very nature of its acts, can proceed only from a simple 

 being, and that consequently it cannot belong to a substance 

 essentially compounded, or at least divisible and decomposable, 

 like matter ; but this metaphysical argument would carry us too 

 far from our subject. We content ourselves with merely sug- 

 gesting it ; for all experimental purposes, it will be sufficient to 

 consider the immateriality of the principle of volition as a dis- 

 tinction founded upon analogy, and the inertia of matter as a 

 general property in the actual state of the world. 



9. We are moreover made acquainted by experiment with 

 several other properties of matter which are also acciden- 

 tal, that is, which seem not to be absolutely necessary in 

 order that material bodies may manifest themselves to our stns- 

 es, but the co- existence of which with the primitive conditions of 

 materiality is important to be known, since it supplies the want of 

 other evidence, in a great number of cases in which the essential 

 properties do not admit of being recognised. Such, for example, 

 is gravity. Among natural bodies which we can see and touch, 

 none is to be found which is not heavy, that is, which does not 

 tend to fall toward the centre of the earth when left to itself; 

 and since these properties are always found to accompany each 

 other, the presence of the one is with respect to us always a 

 sufficient ground to infer the existence of the other. Thus, al- 

 though we can neither see nor touch the air, as we can see and 

 touch other bodies, still we believe it to be a material substance, 

 because it is heavy, capable of being confined in vessels and of 

 exhibiting other phenomena, all similar to those which belong to 

 a heavy fluid. A careful examination of tnese properties teach- 

 es us at length that there are airs of very different kinds, which 

 are all so many substances differing essentially from each other 

 in the action which they are capable of exerting on other bodies, 

 and which is exerted in turn upon them by these bodies. 



10. Moreover attraction is one of those contingent properties 

 which supplies what is wanting in the evidence furnished by the 



