But this is the essence
of all theory. The theory is the backward guess from fact to
principle; the conjecture, or divination regarding something, which
lies behind the facts, and from which they flow in necessary
sequence. If Dalton's theory, then, account for the definite
proportions observed in the combinations of chemistry, its
justification rests upon the same basis as that of the principle of
gravitation. All that can in strictness be said in either case is
that the facts occur as if the principle existed.
The manner in which Faraday himself habitually deals with his
hypotheses is revealed in this lecture. He incessantly employed
them to gain experimental ends, but he incessantly took them down,
as an architect removes the scaffolding when the edifice is complete.
'I cannot but doubt,' he says, 'that he who as a mere philosopher
has most power of penetrating the secrets of nature, and guessing by
hypothesis at her mode of working, will also be most careful for his
own safe progress and that of others, to distinguish the knowledge
which consists of assumption, by which I mean theory and hypothesis,
from that which is the knowledge of facts and laws.' Faraday
himself, in fact, was always 'guessing by hypothesis,' and making
theoretic divination the stepping-stone to his experimental results.
I have already more than once dwelt on the vividness with which he
realised molecular conditions; we have a fine example of this
strength and brightness of imagination in the present 'speculation.
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