tions. Or the observer may perceive it differently, owing to nearsightedness, or color-blindness, or the medium through which it is examined, or even because he has preconceived—that is, prejudiced— opinions of what its appearance ought to be. In short, the judgments we form of things external to us, and even of our internal perceptions, are not invariable nor consistent with one another. They are made up of external realities and internal impressions and conceptions. Color-blindness may be mentioned as a familiar illustration of the errors into which our senses may lead us. It often happens that one cannot distinguish the color of ripened fruit from that of the leaves around it. And certain artist painters who excelled in drawing and composition, owing to color-blindness have produced pictures that were caricatures of nature. The examination of railroad signal-men has showed that this strange defect is not uncommon. But there is also a mental, and there is a moral "colorblindness" which seems to pervert the judgment and the conscience; the good and the true of one person may be the evil and the false of another. Whether this defect be inherited or is the outcome of a vicious education, it lies at the root of the greater number of the differences that divide mankind into nations, and nations into political, and religious, and scientific, and artistic sects. Most of all does it, perhaps, exist in those branches of knowledge which contain a mixture of facts and speculations, and in which, even when the facts are themselves demonstrable, their relations among themselves and to other facts are apt to be differently perceived and estimated by different persons.
It follows, then, that a perfect consensus of opinion upon any subject that is not abstractly or mathematically demonstrable, or that is not imposed by authority, is not to be looked for, and that the most favorable result, as regards practical as well as speculative knowledge, is an enlightened and candid and eager search for truth, a search conducted with all the light furnished by history and experience, with a steadfast adherence and devotion to truth; and, when truth has been found, a stern refusal to be seduced from allegiance to it by any ardent appeal or any plausible arguments offered in the name of liberalism, or any subtle temptations to propose or accept compromises, in the name of uprightness and truth, but really for the promotion of selfish interests.
In addition to all these hindrances to a search for medical and for other truths is the notorious fact that medical literature is full of
