and natural history; and the mere metaphysician who without such 
_ preparation and fitness sets himself to determine the grounds of mathe- 
matical or mechanical truths, or the principles of classification, will be 
__ liable to be led into error at every step. He must speculate by means 
of general terms, which he will not be able to use as instruments of 
discovering and conveying philosophical truth, because he cannot, in 
his own mind, habitually and familiarly, embody their import in special 
examples. 
* Acting upon such views, I have already laid before the Philosophical 
Society” i Cambridge essays on such subjects as I here refer to; espe- 
cially a 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. J am therefore the 
more willing to bring under consideration another subject of a kind 
closely related to the one just mentioned. 
“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 necessary, (which I 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 incongruities into which the opposite false doctrine 
leads us, when applied to particular examples. This accordingly is 
what I propose to do in the present memoir, with regard to the propo- 
Sition stated at the head of this paper, namely, that all matier is heavy. 
“At first sight it may appear a doctrine altogether untenable to as- 
ert that this proposition is a necessary truth: for it may be urged, we 
haye 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 
Pty which hypothesis, however false, shews that such a supposition 
possible. Again, it may be said that weight and inertia are two sep: 
Hse properties of matter ; that mathematicians measure the quantity 
of matter by the inertia, and that we learn _ experiment only that the 
Vol. xt, No, 2.—Jan.-March, 1842. 
