In all our thinking it is very important to get a clear and full vision of the thing to be known. This is not always as easy as it seems. Like Nelson in the battle of Copenhagen, we may consciously turn the blind eye towards what we do not like and exclaim, “I do not see it.” The lenses through which we gaze may be green, or smoked, or ill-adjusted, and thus without suspecting it we may see things in false colors or distorted shapes. Our bodily condition may color everything we see and think. In health and high animal spirits every thought is rose-colored. In periods of disease and depression everything we think seems to pass, “like a great bruise, through yellow, green, blue, purple, to black. A liver complaint causes the universe to be shrouded in gray; and the gout covers it with inky pall, and makes us think our best friends little better than fiends in disguise.”
One of the greatest hinderances to correct thinking is prejudice. Hence all who have presumed to give advice on the conduct of the understanding have had something to say concerning prejudice. Bacon has a chapter on the idols of the mind, and Locke contends that we should never be in love with any opinion. In a charming little volume on the “Art of Thinking,” Knowlson has a chapter in which he enumerates and discusses the prejudices arising from birth, nationality, temperament, theory, and unintelligent conservatism. The list might easily be enlarged. Close analysis must convince any one that feeling strengthens all forms of prejudice, and there are very few, if any, fields of thought in which it is not essential for the attainment of truth to divest ourselves of preconceived notions and the resultant feelings, and to weigh the arguments on both sides of a question before reaching a conclusion.
[Sidenote: The wishes of the heart and the conclusions of the intellect.]
A student may take up geometry with a feeling of prejudice for or against the study, based upon what he has heard from others concerning its difficulties or the teacher who gives the instruction; but after he has mastered the demonstration of a theorem he does not lie awake at night wishing the opposite were true. In the realms of mathematics the wishes of the heart are not in conflict with the conclusions of the intellect. In the domain of ethical, social, historical, or religious truth the head often says one thing and the heart another. “We see plainly enough what we ought to think or do, but we feel an irresistible inclination to think or do something else.” In most of the instances in which the study of science has led to agnosticism the wish was father to the thought. When two men argue the same question, weighing the same arguments and reaching opposite conclusions, as did Stonewall Jackson and his father-in-law at the outbreak of the Civil War, the inclinations and wishes of the heart must have influenced their thinking.