Now, in so far as our conscious mental activity is under the direction of our will, we can improve this form of common sense, as to both its range and the trustworthiness of its judgments, by appropriate training. Such training, as regards the purely intellectual aspect of common sense, will consist in the determinate culture of the habit of honestly seeking for truth,--dismissing prejudice, setting aside self-interest, searching out all that can be urged on each side of the question at issue, endeavouring to assign to every fact and argument its real value, and then weighing the two aggregates against each other with judicial impartiality. For in proportion to the steadiness with which this course is volitionally pursued, must be its effectiveness in shaping the mechanism whose automatic action constitutes the unconscious thinking, of which the results express themselves in our common-sense judgments.
The ordinary common sense of mankind, disciplined and enlarged by an appropriate culture, becomes one of the most valuable instruments of scientific inquiry; affording in many instances the best and sometimes the only basis for a rational conclusion. A typical case, in which no special knowledge is required, is afforded by the flint implements of the Abbeville and Amiens gravel beds. No logical proof can be adduced that the peculiar shapes of these flints were given to them by human hands; but no unprejudiced person who has examined them now doubts it.
The evidence of design to which, after an examination of one or two such specimens, we should only be justified in attaching a probable value, derives an irresistible cogency from accumulation. On the other hand, the improbability that these flints acquired their peculiar shape by accident, becomes to our minds greater and greater as more and more such specimens are found; until at last this hypothesis, although it cannot be directly disproved, is felt to be almost inconceivable, except by minds previously possessed by the dominant idea of the modern origin of man. And thus what was in the first instance a matter of discussion, has now become one of those self-evident propositions, which claim the unhesitating assent of all whose opinion on the subject is entitled to the least weight.