In taking a comprehensive view of any subject, it is very desirable to throw into the shade all its minor points; but in estimating the consequences of any set of facts, there is another condition which must be fulfilled, before we can arrive at accurate conclusions. If we are about to neglect a cause on account of its apparent insignificance, it is _essential_ that it should not be one of _frequent_ recurrence. Thus, if a labourer inconsiderately lift his shovel but an inch or two more than is necessary to throw its load into his barrow, although the exertion of force is trivial in each instance, its repeated occurrence during the whole day, will produce at its conclusion a very sensible difference either in fatigue or in the amount of the work done. Napoleon is said to have remarked of Laplace, when he was Minister of the Interior, that he was too much occupied with considering _les infiniment petites_. To dwell upon small affairs which are isolated, is not the province of a statesman; but to integrate the effect of their constant recurrence is worthy of the greatest. One of the most important processes in all inquiry, is to divide the subject to be considered into as many different questions as it will admit of, and then to examine each separately, or in other words to suppose that each single cause successively varies whilst all the others remain constant. But this most obvious doctrine of common sense has frequently been contested in questions of economical science, and has been often characterized as theoretical, and as entirely inapplicable to the affairs of life. It is certain that very little progress can be made in any subject without this aid, and it is hopeless for those whose minds are incapable of mastering the simpler questions, ever to institute successfully an investigation into their united action. A familiar illustration will explain this better. Two men are making an excavation, removing the earth in the usual way with spades and wheelbarrows. One of these men, Q., does more work than his companion P., and if an inquiry is made, Why is this so? the usual reply would be that Q. is either stronger, more active, or more skilful than P. Now it is the third of these qualifications which is the most important, because if Q. were inferior even both in strength and in activity, he might yet by means of his skill perform a greater quantity of work without fatigue. He might have ascertained that a _given_ weight of earth raised at each shovelfull, together with a certain number of shovelfulls per hour, would be more advantageous for his strength than any other such combination. That a shovel of a certain weight, size, and form would fatigue him less than those of a different construction. That if its handle were two or three inches longer than he required, its additional weight would at the end of the day have been uselessly lifted many hundred times.