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of water as its temperature is reduced near the freezing- 
point, which though undoubtedly no exceptions to the true 
law of causation, yet warn us against admitting any sequence 
the reason of which is unknown, as a necessary law. Of all 
phenomenal laws, the one perhaps of which the evidence 
from observation in favour of its universality seems the most 
complete is the law of gravitation. But if we knew the true 
cause of the phenomenal law that each unit of mass attracts 
every other inversely as the square of the distance, we should 
probably ascertain many conditions under which the results 
would be at variance with what in the present state of our 
knowledge we call the law. For example, Sir W. Thomson 
has pointed out that if Le Sage's explanation were the true 
theory, some crystals might have different weights according 
to the position in which they are held, and that thus work 
might be done by gravity without expenditure of energy ; in 
other words there would be an exception both to the law of 
gravitation and to that of the conservation of energy, as we 
now understand them. 
22. It is, indeed, only in very few cases out of the infinite 
multiplicity of phenomenal laws that science can make even an 
approach to the time law of causation. But even if the wildest 
dreams of modern science were fulfilled, and all such forces as 
elasticity, the attraction of cohesion, electricity, and the like, 
could be exhibited as necessary results of certain combinations 
of mass and motion, — and at present the mathematics are not 
in existence which can accomplish this, yet there would 
still remain the infinitely varied and complex laws of chemical 
forces and agencies, the reasons for which, as far as we can 
judge, lie entirely out of the range of all possible human 
knowledge. Why water, that is a combination of certain pro- 
portions of (what we know as) oxygen and hydrogen, should 
be what it is, and not something else, is a problem which, 
although it refers to a phenomenon the commonest and appa- 
rently the simplest in nature, we cannot, in the present state 
of our knowledge, conceive to be capable of ever receiving such 
a solution as to be intelligible to the human mind. 
23. And we must proceed still further. Even though 
physical science could prove all natural laws to be sequences 
of cause and effect, necessarily determined by the constitution 
of matter as w r e conceive it, this merely removes the difficulty 
a step further from us, and leaves science as incapable as ever 
of proving them to be necessary truths. Our conceptions of 
material substance are nothing more than the generalization 
of effects produced on our senses, but the objective reality may 
be, or rather must be, something quite different. If the 
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