That a stone will
fall, that fire will burn, that the earth has solidity, we have
observed a thousand and a thousand times; and when any new
instance of this nature is presented, we draw without hesitation
the accustomed inference. The exact similarity of the cases gives
us a perfect assurance of a similar event; and a stronger
evidence is never desired nor sought after. But wherever you
depart, in the least, from the similarity of the cases, you
diminish proportionably the evidence; and may at last bring it to
a very weak analogy, which is confessedly liable to error and
uncertainty. After having experienced the circulation of the
blood in human creatures, we make no doubt that it takes place in
T/ITIUS\ and M/AEVIUS\. But from its circulation in frogs and
fishes, it is only a presumption, though a strong one, from
analogy, that it takes place in men and other animals. The
analogical reasoning is much weaker, when we infer the
circulation of the sap in vegetables from our experience that the
blood circulates in animals; and those, who hastily followed that
imperfect analogy, are found, by more accurate experiments, to
have been mistaken.
If we see a house, C/LEANTHES\, we conclude, with the
greatest certainty, that it had an architect or builder; because
this is precisely that species of effect which we have
experienced to proceed from that species of cause. But surely you
will not affirm, that the universe bears such a resemblance to a
house, that we can with the same certainty infer a similar cause,
or that the analogy is here entire and perfect.
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