640 FRAGMENTS OF SCIENCE. 
The territory of physics is wide, but it has its limits from 
which we look with vacant gaze into the region beyond. 
Let us follow matter to its utmost bounds, let us claim it 
in all its forms even in the muscles, blood, and brain of 
man himself as ours to experiment with and to speculate 
upon. Casting the term " vital force "from our vocab- 
ulary, let us reduce, if we can, the visible phenomena of 
life to mechanical attractions and repulsions. Having 
thus exhausted physics, and reached its very rim, a mighty 
Mystery still looms beyond us. We have, in fact, made 
no step toward its solution. And thus it will ever loom, 
compelling the philosophies of successive ages to confess 
that 
" ' We are such stuff 
As dreams are made of, and our little life 
Is rounded by a sleep.' " 
In my work on " Heat," published in 1863 and republished 
many times since, I employ the precise language thus 
extracted from the Saturday Review. 
The distinction is here clearly brought out which I had 
resolved at all hazards to draw that, namely, between 
what men knew or might know, and what they could never 
hope to know. Impart simple magnifying power to our 
present vision, and the atomic motions of the brain itself 
might be brought into view. Compare these motions with 
the corresponding states of consciousness, and an empirical 
nexus might be established; but "we try to soar in a 
vacuum when we endeavor to pass by logical deduction 
from the one to the other." Among these brain-effects a 
new product appears which defies mechanical treatment. 
We cannot deduce motion from consciousness or conscious- 
ness from motion as we deduce one motion from another. 
Nevertheless observation is open to us, and by it relations 
may be established which are at least as valid as those of 
the deductive reason. The difficulty may really lie in the 
attempt to convert a datum into an inference an ultimate 
fact into a product of logic. My desire for the moment, 
however, is not to theorize, but to let facts speak in reply 
to accusation. 
The most " materialistic " speculation for which I was 
responsible, prior to the " Belfast Address," is embodied 
in the following extract from a brief article written as far 
back as 1865: '-'Supposing the molecules of the human 
