e in the moral sciences, owing to the immense
multitude of the influencing circumstances, and our very scanty means of
varying the experiment. Even in operating upon an individual mind, which
is the case affording greatest room for experimenting, we cannot often
obtain a _crucial_ experiment. The effect, for example, of a particular
circumstance in education, upon the formation of character, may be tried
in a variety of cases, but we can hardly ever be certain that any two of
those cases differ in all their circumstances except the solitary one of
which we wish to estimate the influence. In how much greater a degree
must this difficulty exist in the affairs of states, where even the
_number_ of recorded experiments is so scanty in comparison with the
variety and multitude of the circumstances concerned in each. How, for
example, can we obtain a crucial experiment on the effect of a
restrictive commercial policy upon national wealth? We must find two
nations alike in every other respect, or at least possessed, in a degree
exactly equal, of everything which conduces to national opulence, and
adopting exactly the same policy in all their other affairs, but
differing in this only, that one of them adopts a system of commercial
restrictions, and the other adopts free trade. This would be a decisive
experiment, similar to those which we can almost always obtain in
experimental physics. Doubtless this would be the most conclusive
evidence of all if we could get it. But let any one consider how
infinitely numerous and various are the circumstances which either
directly or indirectly do or may influence the amount of the national
wealth, and then ask himself what are the probabilities that in the
longest revolution of ages two nations will be found, which agree, and
can be shown to agree, in all those circumstances except one?

Since, therefore, it is vain to hope that truth can be arrived at,
either in Political Economy or in any other department of the social
science, while we look at the facts in the concrete, clothed in all the
complexity with which nature has surrounded them, and endeavour to
elicit a general law by a process of induction from a comparison of
details; there remains no other method than the _a priori_ one, or that
of "abstract speculation."

Although sufficiently ample grounds are not afforded in the field of
politics, for a satisfactory induction by a comparison of the effects,
the causes may, in all cases