The hypothesis maker has a specific form of this fear of inconsistency. This type of theorist makes a supposition to account for certain facts. When he meets with certain allied facts for which the supposition apparently does not account, he either ignores said facts, or cuts and trims them, or bullies them into his theory. Hypotheses _per se_ have never done any harm. In fact they are indispensable in all thought, especially as an aid to observation. But it is the desire to prove an hypothesis correct, simply because it is _our_ hypothesis, or because it is a fascinating hypothesis, which has done harm. Darwin says that he had made it a habit “whenever a published fact, a new observation or thought came across me, which was opposed to my general results, to make a memorandum of it without fail and at once; for I had found by experience that such facts and thoughts were far more apt to escape from the memory than favorable ones.” The second reason for desiring to cling to an opinion because we already hold it is one which could probably best be explained by physiological psychology and a study of the brain. We feel almost a physical pain when a tenet we have long cherished is torn up and exposed. The longer we hold an opinion, the harder it is for us to get rid of it. In this respect it is similar to habit. Nor is the comparison an analogy merely. An opinion is a habit of thought. It has the same basis in the brain, and is subject to the same laws, as a habit of action. It is well known that the opinions of a man over forty are pretty well set. The older a man grows, the harder it is for him to change an opinion—or for others to change it for him. The side of a controversy we see first is usually the side we see last. This is because the arguments we meet do not have to shake up or dislodge anything in our brain (unless we are very critical, and we generally aren’t). But once let an opinion gain entrance, and any opinion contrary to it will have to dislodge the old one before it can find a place for itself.