MAXIM I. Where there is no real doubt there can be no real investigation. This seems to be sufficiently obvious, and it is difficult to find any clearer truth by which to illustrate it. Yet it is often forgotten. For example the Cartesian method of philosophizing is to begin with a state of philosophic doubt and requires us to lay aside all our beliefs and begin the whole process of inference anew. Now there never would have been any Cartesians in the world if it had been understood that this philosophic doubt must be genuine doubt, and if students had had any proper self-knowledge. It is plainly impossible to have an unaffected doubt that fire burns,--and one which will resist a few experiments,--unless one is incapable of reasoning.
MAXIM II. What is questioned by instructed persons is not certain. If two men think differently either may be right; and that one of them is I makes no difference for each is in the first person to himself. If a demonstration appears perfectly conclusive to one person and not so to another, it may be that there is some fallacy in it. Nevertheless, the opinions of most persons upon most subjects may be entirely neglected. A child's judgment of a lover's motives should have as much weight against a grown person's as the judgment of an ordinary person of intelligence against that of a man who is peculiarly fitted by natural bent, severe training, and large experience, for judging of the subject. The belief in the right to a private opinion which is the essence of protestantism, is carried to a ridiculous excess in our community. Some years ago, an instrument was invented called Hedgecock's Quadrant, by observing a candle with which, as the heavenly bodies are observed with an ordinary quadrant, it was pretended that the latitude and longitude could both be ascertained. Most of the newspapers and several ship-owners thought it a valuable invention; but physicists would not listen to their arguments. It was generally thought that this was very wrong, but the event has justified them.
On the other hand, it is folly in me not to doubt what men as capable as myself of forming a correct conclusion doubt. For Agassiz to attach no weight to the opinion of Darwin or for Darwin to attach no weight to that of Agassiz, would show a narrow-mindedness, most fatal to the sober investigation of truth. No self-evident proposition is more recklessly disregarded than this second maxim. We often hear such terms as indubitable applied to propositions which actually are doubted by a large proportion of experts; and such language certainly argues great intemperance and want of discipline in him who uses it.
If anybody objects to this that the object of reasoning is rather to ascertain the truth than to make the peace between disputants, I agree with him entirely. We wish to ascertain the truth, but what is truth? This is an indispensible inquiry if we so define the function of reason, yet it would plunge us at once into a sea of metaphysics from which we could not hope soon to emerge. Opinions upon this subject are various; and it is therefore uncertain what truth is. It is not likely we could reconcile those opinions when so many greater men have failed, and therefore we could not obtain any certain answer to this question. By such a method, therefore, we could gain no clear and trustworthy conception of the end of reasoning. Let us then avoid this idea of truth as long as we can and keep in the realm of those everyday and concrete notions about which there can be no mystery nor vagueness.
Any useful inquisition must lead to some definite conclusion, for a method of investigation which should carry different men to different results without tending to bring them to agreement, would be self-destructive and worthless. Consequently, reasoning rightly conducted does tend to produce an agreement among men; and doubt once dispelled investigation must cease. Our maxim, therefore, defines in some degree at least the end of reasoning; that is, it serves to exclude a part of those things which are inconsistent with the true end. We shall see presently what needs to be added to this rule.
This is a maxim constantly neglected. Some persons seem to think the chief use of the power of reasoning is its own exercise. And so they make the object of the process the keeping up of a disputation instead of the bringing of it to a close. The best cure for such a spirit of disputatiousness is the constant practical application of reason where its inferences will be speedily tested, and especially the study of the natural sciences. According to my observation there is not one out of two hundred of those of our graduates who have any intellectual force who escape this cursed disease which has always infected schools. And, therefore, in my opinion the first thing a graduate should do is to put himself under the care of a first-rate teacher in a science of observation.