A question has perhaps just occurred to the reader. If the deductive method is to be checked up by experiment, and the results of the experiment are always to be taken, why not experiment first, and omit theory altogether?

Leaving aside the fact that theory is the best guide for experiment—that were it not for theory and the problems and hypotheses that come out of it, we would not know the points we wanted to verify, and hence would experiment aimlessly—a more serious objection is that experiment is seldom if ever perfect, for it nearly always involves some unverified assumption. I have referred to empirical observation and experiment as two different methods. But the difference is mainly, if not solely, one of degree. If we experimented to find out whether acquired char­ac­ter­is­tics were inherited, it is obvious that our experiments would have to be confined to animals. If we found, let us say, that no acquired char­ac­ter­is­tic was ever transmitted to offspring, we could not say that this would be equally true of man, but would be justified in concluding only that the acquired char­ac­ter­is­tics of animals are not transmitted to descendants. Nay, we could not go even this far. We would have to confine ourselves to the statement that certain acquired char­ac­ter­is­tics of the few score animals we had experimented upon were not transmissible. But even this statement would involve assumption. We could say only that certain acquired char­ac­ter­is­tics of the few score animals we had experimented upon had not been transmitted in these par­tic­u­lar instances. We would have to limit ourselves to a bare statement of fact; we could draw no conclusion whatever. But if we had attacked this problem from the deductive standpoint, and had concluded that owing to certain conditions holding alike in all animals and in man, acquired char­ac­ter­is­tics could not possibly be transmitted, we would have sufficient ground for deriving from our experiments a broad gen­er­al­i­za­tion.

Experiment and deduction are not the only methods which can be checked up against each other. We can do likewise with the comparative and the experimental, the historical and the theoretical—in fact, all viewpoints applicable to any one problem.

When you encounter a question about which there is a controversy, and where the adherents of both sides nearly equal each other in number and in­tel­lec­tual status, you may be almost certain that each side has caught sight of some truth, but that neither has seen the whole truth; and you should endeavor to unite both sides by a broader and deeper solution. A classic philosophical example of this method is Herbert Spencer’s attempt to reconcile science and religion, and his effort to unite the “intuitional” and “experiential” schools of thought. The intuitionists maintained that the mind had from birth intuitions by which it knew certain truths in­de­pen­dently of experience. Such truths as the axiom that a straight line is the shortest distance between two points, or that it is morally wrong to do certain acts, were regarded as among these intuitions. The “empiricists” or “sensa­tion­alists,” on the other hand, maintained that all our knowledge—even of such a fact, for instance, as that two and two are four, where we cannot conceive otherwise—is learned solely from the individual’s experience, taken in its broadest sense. Herbert Spencer thought he recognized some truth in both these doctrines, and came forward with the theory that there are certain truths which are intuitions so far as the individual is concerned, but that these intuitions have been inherited from our ancestors, were originally built up through the ages, and represent the accumulated experience of the race. Whatever may be thought of Spencer’s success in this case, the value of the method itself is undoubted. It was frequently used by Kant, Hegel, Fichte and other German philosophers.

I have remarked that it is almost possible to sum up the entire process of thinking as the occurrence of sug­ges­tions for the solution of difficulties and the testing out of those sug­ges­tions. The constructive methods discussed were called means for making good sug­ges­tions occur to us. From this standpoint the cautions with which we have just been dealing may be considered as tests of sug­ges­tions.

Let us refer back to the analysis of thinking given in the case of the man who discovered footprints on the beach. Even there, in order to give any adequate idea of his thought process, I was obliged to show that for various reasons he rejected certain suggested solutions. But this negative method could be more fully developed. Because the man rejected a certain solution, it does not follow that it was necessarily wrong. Suppose the final sug­ges­tion—that the unknown had been on the island all the time—were to have been tested out, and that certain further facts were discovered which tended to disprove it; the man might find it necessary to look for still another solution. But suppose this were not forthcoming, suppose that all the possibilities had been exhausted. It would be necessary to return to some of the original sug­ges­tions. He would have to see whether an error had been made in testing them. In rejecting the sug­ges­tion of a small boat he may have overestimated the distance of this island from other land. He may have underestimated the difficulties that a man in a small boat is capable of surmounting. In rejecting the sup­po­si­tion of a ship, he may have erred in his judgment of the time the footprints had been on the beach, or of the time it would take a large vessel to get out of sight.

What is essential is that all sug­ges­tions be tested out, either by memory, observation or experiment, in all their implications, and that the tendency be resisted to accept the first solution that suggests itself. For the uncritical thinker will always jump at the first sug­ges­tion, unless an objection actually forces itself into view. Remaining in a state of doubt is unpleasant. The longer the doubt remains the more unpleasant it becomes. But the man who is willing to accept this unpleasantness, the man who is willing carefully to observe, or experiment if need be, to test the validity of his sug­ges­tions, will finally arrive at a solution much deeper, and one which will give him far more satisfaction, than the superficial answer obtained by the man of careless habits of thought.

Thomas A. Edison says he always rejects an easy solution of any problem and looks for something difficult. But the inventor has one great advantage over any other kind of thinker. He can test his conclusion in a tangible way. If his device works, his thinking was right; if his device doesn’t work, his thinking was wrong. But the philosopher, the scientist, the social reformer, has no such satisfactory test. His only satisfaction is the feeling that his results harmonize with all his experience. The more critical he has been in arriving at those results, the more deep and permanent will be that feeling, the more valuable will be his thoughts to himself and to the world. . . .

Even in the first chapter I intimated that logic would constitute a part of the science of thinking. I intimated, moreover, that it would constitute almost the whole of what may be called the negative side of thinking—those rules which serve to steer thought aright. Though cautionary, the advice given in this chapter is not usually given in books on logic. But though I cannot overemphasize the importance of a knowledge of logic, I cannot deal with it here. The science can receive justice only in a book devoted entirely to it.