Of those departments of science, on the other hand, in which our conclusions rest (like those of ordinary common sense) not on any one set of experiences, but upon our unconscious co-ordination of the whole aggregate of our experiences,--not on the conclusiveness of any one train of reasoning, but on the convergence of all our lines of thought towards one centre,--geology may be taken as a typical example. For this inquiry brings (as it were) into one focus, the light afforded by a great variety of studies,--physical and chemical, geographical and biological; and throws it on the pages of that great stone book in which the past history of our globe is recorded. And its real progress dates from the time when that common sense method of interpretation came to be generally adopted, which consists in seeking the explanation of past changes in the forces at present in operation, instead of invoking (as the older geologists were wont to do) the aid of extraordinary and mysterious agencies.
Of the adequacy of common sense to arrive at a decisive judgment under the guidance of the convergence just indicated, we have a good example in the following occurrence:--A man having had his pocket picked of a purse, and the suspected thief having been taken with a purse upon him, the loser was asked if he could swear to it as his property. This he could not do; but as he was able to name not only the precise sum which the purse contained, but also the pieces of money of which that sum consisted, the jury unhesitatingly assigned to him the ownership of the purse and its contents. A mathematician could have calculated, from the number of coins, what were the chances against the correctness of a mere guess; but no such calculation could have added to the assurance afforded by common sense, that the man who could tell not only the number of coins in the purse, but the value of each one of them, must have been its possessor.
Familiar instances of the like formation of a basis of judgment by the unconscious co-ordination of experiences, will be found in many occurrences of daily life; in which the effect of special training manifests itself in the formation of decisions, that are not the less to be trusted because they do not rest on assignable reasons:--Thus, a literary man, who has acquired by culture the art of writing correctly and forcibly, without having ever formally studied either grammar, the logical analysis of sentences, or the artifices of rhetoric, will continually feel in criticizing his own writings or those of others, that there is something faulty in style or construction, and may be able to furnish the required correction, whilst altogether unable to say in what the passage is wrong, or why his amendment sets it right. Or, to pass into an entirely different sphere, a practised detective will often arrive, by a sort of divination, at a conviction of the guilt or innocence of a suspected person, which ultimately turns out to be correct; and yet he could not convey to another any adequate reasons for his assurance, which depends upon the impression made upon his mind by minute details of look, tone, gesture, or manner, which have little or no significance to ordinary observers, but which his specially cultured common sense instinctively apprehends.