causes are entirely similar, the analogy is perfect, and the inference,
drawn from it, is regarded as certain and conclusive: nor does any man
ever entertain a doubt, where he sees a piece of iron, that it will have
weight and cohesion of parts; as in all other instances, which have ever
fallen under his observation. But where the objects have not so exact a
similarity, the analogy is less perfect, and the inference is less
conclusive; though still it has some force, in proportion to the degree
of similarity and resemblance. The anatomical observations, formed upon
one animal, are, by this species of reasoning, extended to all animals;
and it is certain, that when the circulation of the blood, for instance,
is clearly proved to have place in one creature, as a frog, or fish, it
forms a strong presumption, that the same principle has place in all.
These analogical observations may be carried farther, even to this
science, of which we are now treating; and any theory, by which we
explain the operations of the understanding, or the origin and connexion
of the passions in man, will acquire additional authority, if we find,
that the same theory is requisite to explain the same phenomena in all
other animals. We shall make trial of this, with regard to the
hypothesis, by which we have, in the foregoing discourse, endeavoured
to account for all experimental reasonings; and it is hoped, that this
new point of view will serve to confirm all our former observations.

83. _First_, It seems evident, that animals as well as men learn many
things from experience, and infer, that the same events will always
follow from the same causes. By this principle they become acquainted
with the more obvious properties of external objects, and gradually,
from their birth, treasure up a knowledge of the nature of fire, water,
earth, stones, heights, depths, &c., and of the effects which result
from their operation. The ignorance and inexperience of the young are
here plainly distinguishable from the cunning and sagacity of the old,