hat chances
to pass before us, when either we are idle or engaged with some other
inquiry, and so obtain a new glimpse of the course of nature; or we may
try experiments haphazard, and watch the results. But, even in these
cases, before our new notions can be considered knowledge, they must be
definitely framed in hypotheses and reobserved or experimented upon,
with whatever calculations or precautions may be necessary to ensure
accuracy or isolation. As a rule, when inquiring deliberately into the
cause of an event, whether in nature or in history, we first reflect
upon the circumstances of the case and compare it with similar ones
previously investigated, and so are guided by a preconception more or
less definite of 'what to look for,' what the cause is likely to be,
that is, by an hypothesis. Then, if our preconception is justified, or
something which we observe leads to a new hypothesis, either we look for
other instances to satisfy the canons of Agreement; or (if the matter
admits of experiment) we endeavour, under known conditions according to
the canon of Difference, to reproduce the event by means of that which
our hypothesis assigns as the cause; or we draw remote inferences from
our hypothesis, and try to test these by the Inductive Canons.

If we argue from an hypothesis and express ourselves formally, it will
usually appear as the major premise; but this is not always the case. In
extending ascertained laws to fresh cases, the minor premise may be an
hypothesis, as in testing the chemical constitution of any doubtful
substance, such as a piece of ore. Some solution or preparation, A, is
generally made which (it is known) will, on the introduction of a
certain agent, B, give a reaction, C, if the preparation contains a
given substance, X. The major premise is the law of reaction--

Whenever A is X, if treated with B it is C.

The minor premise is an hypothesis that the preparation contains X. An
experiment then treats A with B. If C result, a probability is raised in
favour of the hypothesis that A is X; or a certainty, if we know that C
results on that condition only.

So important are hypotheses to science, that Whewell insists that they
have often been extremely valuable even though erroneous. Of the
Ptolemaic system he says, "We can hardly imagine that Astronomy could,
in its outset, have made so great a progress under any other form." It
served to connect men's thoughts on the subject and to susta