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 _à 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 _à 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 arise, these might always be brought within the pale of the abstract science if it were worth while; and when we make the necessary allowances for them in practice, if we are doing anything but guess, we are following out the method of the abstract science into minuter details; inserting among its hypotheses a fresh and still more complex combination of circumstances, and so adding _pro hác vice_ a supplementary chapter or appendix, or at least a supplementary theorem, to the abstract science.