June, 1889. 
OUR PHYSICAL BELIEFS. 
135 
I imagine that the bone of contention is the question 
whether physical truths can be known a 'priori or not ? That 
the Professor takes the latter view is evident from his closing 
sentence which runs thus : “ In fact 1 suspect that the mind is 
provided only with machinery ready to arrange the results put 
into it by the senses, and that it does not contain any results 
ready made.” 
Contrary as this is to all metaphysical doctrines by what 
school soever held, it does not concern us here ; all that is 
desired is to prove it erroneous in its application to physics. 
Mr. Spencer, when replying to the “ Quarterly Review ” 
which attacked him on this point, October, 1873, so thoroughly 
threshed out the matter that nothing I could possibly add 
(even were I so presumptuous) could do otherwise than 
weaken my case. Mr. Spencer took as his premises a 
quotation from his critic’s chief authority (Thomson and Tait). 
It was as follows : “ Physical axioms are axiomatic to those 
only who have sufficient knowledge of the action of physical 
causes to enable them to see at once their necessity.” What 
do we understand by an axiom ? Clearly an a priori truth. If 
not, by what process of experiment do you proceed to prove 
an axiom such as that expressed by the words, “ Things that 
are equal to the same are equal to one another? ” It cannot 
be done, as at every step of the proof it is taken for granted; 
presuming that physical axioms partake of the same 
character as those of mathematics. Again the phrase, “ To 
see at once their necessity,” also excludes the a posteriori view. 
Mr. Spencer says : “ Though Newton gives illustrations of 
prolonged motion in bodies that are little resisted, he gives no 
proof that a body in motion will continue moving if 
uninterfered with, in the same direction at the same velocity.” 
“ Does Professor Tait deny that the first law of motion is a 
physical truth, and denying that it is established a posteriori — 
that is, by conscious induction from observation and 
experiment? If so, what is the inductive reasoning which 
can establish it ? ” 
I should like to refer to the opinion of another physicist, 
Professor Tilden. The practice of pitting authority against 
authority is often objectionable, but the point he refers to in 
this connection is of sufficient interest to excuse it. Speaking 
of the Molecular Hypothesis, and comparing it with the 
theory of Gravitation, he says We possess at present no 
direct or positive proof even that molecules exist, still less 
have we any evidence regarding the conditions under which 
they may subsist in mass. In neither case 
does the theory admit of direct experimental proof ; but both are 
