Locke seems to confound certainty with assurance in his forcible description of the way in which men are guided in their daily affairs (Human Understanding, iv. 14, par. 1):
Man would be at a great loss if he had nothing to direct him but what has the certainty of true knowledge. For that being very short and scanty, he would be often utterly in the dark, and in most of the actions of his life, perfectly at a stand, had he nothing to guide him in the absence of clear and certain knowledge. He that will not eat till he has demonstration that it will nourish him, he that will not stir till he infallibly knows the business he goes about will succeed, will have little else to do than to sit still and perish.
A society may be considered as a highly complex organism, with a consciousness of its own, caring only for itself, establishing regulations and customs for its collective advantage, and creating a code of opinions to subserve that end. It is hard to over-rate its power over the individual in regard to any obvious particular on which it emphatically insists. I trust in some future time that one of those particulars will be the practice of Eugenics. Otherwise the influence of collective truths on individual conduct is deplorably weak, as expressed by the lines:—
Professor Westermarck, among many other remarks in which I fully concur, has aptly stated (Sociological Papers, published for the Sociological Society. Macmillan, 1906, vol. ii., p. 24), with reference to one obstacle which prevents individuals from perceiving the importance of Eugenics, ‘the prevalent opinion that almost anybody is good enough to marry is chiefly due to the fact that in this case, cause and effect, marriage and the feebleness of the offspring, are so distant from each other that the near-sighted eye does not distinctly perceive the connexion between them.’ (The Italics are mine.)