ail,
of every part in a complicated appearance, would be endless and
insignificant. When, however, any question in a theory depends upon the
nature of an appearance, we cannot be too particular in describing that
by which the question is to be decided. But though it be sometimes
proper to be minute in a particular, it is always, and above all things,
necessary to be distinct; and not to confound together things which are
of different natures. For, though it be by finding similarity, in things
which at first sight may seem different, that science is promoted and
philosophy attained, yet, we must have a distinct view of those things
which are to be assimilated; and surely the lowest state of knowledge
in any subject, is the not distinguishing things which, though not to
common observation different, are not truly the same.

To confound, for example one stone with another, because they were both
hard, friable, and heavy, would be to describe, with the superficial
views of vulgar observation; whereas science specifies the weight and
hardness, and thus accurately distinguishes the stone.

Before naturalists had learned to distinguish what they saw, and to
describe, in known terms, those natural appearances, a theorist must
have generalised only from his proper observation. This has been my
case. When I first conceived my theory, few naturalists could write
intelligibly upon the subject; but that is long ago, and things are much
altered since; now there are most enlightened men making observations,
and communicating natural knowledge. I have the satisfaction, almost
every day, to compare the theory, which I had formed from my proper
observations, with the actual state of things in almost every quarter of
the globe.

Whether, therefore, we mean to try a theory by its application to
such phenomena as are well understood, or to learn something from the
application of particular phenomena to a well established theory,
we shall always find it interesting to have appearances described;
particularly such as may be referred to some general rule, as
circumscribing it to certain conditions, or as finding rule in rule,
that is to say, discovering those particular conditions in which the
general laws of action may be affected.

Instead, for example, of the rule which we find in the application of
heat for the fusion and evaporation of mineral substances upon the
surface of this earth, we may find it necessary to consider the effect