198 PROFESSOR WHEWELL's DEMONSTRATION 



memoir " On the Nature of the Truth of the Laws of Motion," which was 

 printed by the Society in its Transactions. This memoir appears to have 

 excited in other places, notice of such a kind as to shew that the minds of 

 many speculative persons are ready for and inclined towards the discussion 

 of such questions. I am therefore the more willing to bring under con- 

 sideration another subject of a kind closely related to the one just men- 

 tioned. 



The general questions which all such discussions suggest, are (in the 

 existing phase of English philosophy) whether certain proposed scientific 

 truths, (as the laws of motion,) be necessary truths ; and if they are neces- 

 sary, (which 1 have attempted to shew that in a certain sense they are,) on 

 what ground their necessity rests. These questions may be discussed in a 

 general form, as I have elsewhere attempted to shew. But it may be 

 instructive also to follow the general arguments into the form which they 

 assume in special cases ; and to exhibit, in a distinct shape, the incongrui- 

 ties into which the opposite false doctrine leads us, when applied to par- 

 ticular examples. This accordingly is what I propose to do in the present 

 memoir, with regard to the proposition stated at the head of this paper, 

 namely, that all matter is heavy. 



At first sight it may appear a doctrine altogether untenable to assert 

 that this proposition is a necessary truth : for, it may be urged, we have no 

 difficulty in conceiving matter which is not heavy ; so that matter without 

 weight is a conception not inconsistent with itself; which it must be if the 

 reverse were a necessary truth. It may be added, that the possibility of 

 conceiving matter without weight was shewn in the controversy which 

 ended in the downfall of the phlogiston theory of chemical composition ; 

 for some of the reasoners on this subject asserted phlogiston to be a body 

 with positive levity instead of gravity, which hypothesis, however false, 

 shews that such a supposition is possible. Again, it may be said that 

 weight and inertia are two separate properties of matter : that mathemati- 

 cians measure the quantity of matter by the inertia, and that we learn by 

 experiment only that the weight is proportional to the inertia ; Newton's 

 experiments with pendidums of different materials having been made with 

 this very object. 



