4 
to constitute it in its entirety one of these parts is as indis- 
pensable as the other. In order to maintain this position 
completely, it would be requisite to go over the same ground 
as that occupied by the major part of the paper “ On the 
Metaphysics of Scripture,” which I submitted to the Institute 
on May 1 of last year. For my present purpose it may suffice 
to recapitulate some of the arguments adduced in that paper, 
and to cite others by reference to the numbers prefixed to the 
paragraphs in which they are contained. 
6. It will be found that in that communication I have dis- 
tinguished between the two departments of physics by saying, 
that one part wholly consists in the discovery of facts and 
laws by means of experiment and observation, and the other 
in accounting for the facts and laws by mathematical reasoning 
founded on certain antecedent premisses. Reference was made, 
for illustration, to the scientific labours of Galileo, Kepler, and 
Newton. It was argued that Newton's calculation of the 
movements produced by the action of forces on material par- 
ticles, was not possible till Galileo had certified by experiment 
the parabolic motion of a projectile acted upon by terrestrial 
gravity. And again, after Newton had discovered how to # 
calculate the effects of an attractive force emanating from a 
centre (a vast achievement), and had proved abstractedly, on 
the hypothesis that the force diminished with distance accord- 
ing to the law of the inverse square, that a particle of matter 
under its influence would describe a conic section, the result 
would have been barren and simply speculative, unless ob- 
servations, such as those of Tycho Brahe and Kepler, had 
shown that the elliptic movement was a physical reality. This 
is an instructive instance of the mutual relation between the 
parts respectively performed by observation and by theoretical 
reasoning. It is obvious that we know more about the move- 
ments of the planets than could have been gathered from the 
results of Kepler's labours, because from these alone it was 
not possible to learn whether, or in what manner, the move- 
ments were determined by the action of force. Newton's 
reasoning not only accounted for the elliptic motion, but also 
indicated that it was caused by force acting in an ascertained 
definite manner : the Newtonian theory of gravitation appears 
to have exhibited the very first instance of a fact of nature 
being demonstratively ascribed to a causative antecedent. 
6*. It is a distinguishing characteristic of the theoretical 
department of physical science, that the reasoning it requires 
is always and necessarily founded on hypotheses. The reason 
for this necessity is, that the very purpose of theoretical in- 
vestigation is to ascertain the truth or untruth of hypotheses 
