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extended, to get rid of all unknown forces, and to explain 
them as modes of motion which produce other motions as the 
necessary consequence of matter being inert and occupying 
space. I need merely refer to the modern theories of light 
and heat and molecular action, and, above all, to the vortex 
atom theory, as illustrations of this continual and ever- 
increasing tendency of true physical science. 
20. But it will be needful to consider a little more carefully 
this very important principle, that no law of force, indeed no 
physical law whatever, can be accepted as a necessary truth, 
unless it can be exhibited as a sequence of cause and effect, 
the reason of which is known. Because if the reason be 
unknown there is no security whatever that the same antece- 
dents will always be followed by the same event. I am glad 
here to use the language of so acute a logician as Professor 
Jevons, who in his preface to his treatise on logic and 
scientific method, expresses his “ strong conviction that before 
a rigorous logical scrutiny the reign of law will prove to be 
an unverified hypothesis, the uniformity of nature an ambiguous 
expression, the certainty of our scientific inferences to a great 
extent a delusion.” In that work he argues that “no expe- 
rience of finite duration can be expected to give an exhaustive 
knowledge of all the forces which are in operation. There is 
thus a double uncertainty ” as to the uniformity of natural 
laws. “ Even supposing the universe as a whole to proceed 
unchanged, we do not know the universe as a whole. Com- 
paratively speaking, we know only a point in its infinite 
extent, and a moment in its infinite duration. We cannot be 
sure then that some fact has not escaped our observation, 
which will cause the future to be apparently different from 
the past ; nor can we be sure that the future will really be 
the outcome of the past/’ (Principles of Science, vol.i. p. 169.) 
21. It appears then that the tendency of the human mind 
to accept as necessary laws sequences which, within the 
limits of human experience, are found to bo uniform, but the 
causation of which is unknown, is not an obedience to reason, 
but rather a subjection to sense. The recognition of the 
uniformity, and the classification of apparently diverse pheno- 
mena as results of one natural law, are in themselves triumphs 
of reason over sense ; but when it is further supposed that 
the phenomenal laws thus established, by an induction neces- 
sarily imperfect, are safe from exceptions or even reversal, 
this is to follow the suggestions of the senses and to abandon 
the guidance of reason. In fact, there are found not unfre- 
quently what seem to us in our ignorance arbitrary excep- 
tions to phenomenal laws, such, for example, as the expansion 
