ate the phenomenon by placing it among known
circumstances, which can be then infinitely varied by introducing a
succession of well-defined new ones.

Observation cannot ascertain the effects of a given cause, because it
cannot, except in the simplest cases, discover what are the concomitant
circumstances; and therefore sciences in which experiment cannot be
used, either at all, as in astronomy, or commonly, as in mental and
social science, must be mainly deductive, not inductive. When, however,
the object is to discover causes by means of their effects, observation
alone is primarily available, since new effects could be artificially
produced only through their causes, and these are, in the supposed case,
unknown. But even then observation by itself cannot directly discover
causes, as appears from the case of zoology, which yet contains many
recognised uniformities. We have, indeed, ascertained a real uniformity
when we observe some one antecedent to be invariably found along with
the effects presented by nature. But it is only by reversing the
process, and experimentally producing the effects by means of that
antecedent, that we can prove it to be unconditional, i.e. the cause.

CHAPTER VIII. AND NOTE TO CHAPTER IX.[1]

THE FOUR METHODS OF EXPERIMENTAL ENQUIRY.

Five canons may be laid down as the principles of experimental enquiry.
The first is that of the Method of Agreement, viz.: _If two or more
instances of the phenomenon under investigation have only one
circumstance in common, the circumstance in which alone all the
circumstances agree is the cause or the effect of the given phenomenon._
The second canon is that of the Method of Difference, viz.: _If an
instance in which the phenomenon occurs and an instance in which it does
not occur have every circumstance in common, save one, and that one
occurs only in the former, that one circumstance is the effect, or the
cause, or a necessary part of the cause, of the phenomenon._

These two are the simplest modes of singling out from the facts which
precede or follow a phenomenon, those with which it is connected by an
invariable law. Both are methods of elimination, their basis being, for
the method of agreement, that whatever can be eliminated _is not_, and
for that of difference, that whatever cannot be eliminated _is_
connected with the given phenomenon by a law. It is only, however, by
the method of difference, which is a method of artificial experiment