nters into the process--an uncertainty inherent in the nature of these
complex phenomena, and arising from the impossibility of being quite
sure that all the circumstances of the particular case are known to us
sufficiently in detail, and that our attention is not unduly diverted
from any of them.

This constitutes the only uncertainty of Political Economy; and not of
it alone, but of the moral sciences in general. When the disturbing
causes are known, the allowance necessary to be made for them detracts
in no way from scientific precision, nor constitutes any deviation from
the _a priori_ method. The disturbing causes are not handed over to be
dealt with by mere conjecture. Like _friction_ in mechanics, to which
they have been often compared, they may at first have been considered
merely as a non-assignable deduction to be made by guess from the result
given by the general principles of science; but in time many of them are
brought within the pale of the abstract science itself, and their effect
is found to admit of as accurate an estimation as those more striking
effects which they modify. The disturbing causes have their laws, as the
causes which are thereby disturbed have theirs; and from the laws of the
disturbing causes, the nature and amount of the disturbance may be
predicted _a priori_, like the operation of the more general laws which
they are said to modify or disturb, but with which they might more
properly be said to be concurrent. The effect of the special causes is
then to be added to, or subtracted from, the effect of the general ones.

These disturbing causes are sometimes circumstances which operate upon
human conduct through the same principle of human nature with which
Political Economy is conversant, namely, the desire of wealth, but which
are not general enough to be taken into account in the abstract science.
Of disturbances of this description every political economist can
produce many examples. In other instances the disturbing cause is some
other law of human nature. In the latter case it never can fall within
the province of Political Economy; it belongs to some other science; and
here the mere political economist, he who has studied no science but
Political Economy, if he attempt to apply his science to practice, will
fail. [11]

As for the other kind of disturbing causes, namely those which operate
through the same law of human nature out of which the general principles
of the science aris