We may illustrate this in a very simple manner. One of the most striking differences between the man of to-day and the man of the past is the attitude of the two in relation to natural phenomena. To the people of not so many generations ago an eclipse was a very serious thing, fraught with the promise of disaster to mankind. The appearance of a comet was no less ominous. John Knox saw in comets an indication of the wrath of heaven, and in all countries the Churches fought with all their might against the growth of the scientific view. Away back in antiquity we meet with the same view. There is, for example, the classic case of the Greek general Nikias, who, when about to extricate his army from a dangerous position before Syracuse, was told that an eclipse of the sun indicated that the gods wished him to stay where he was for three times nine days. Nikias obeyed the oracles with the result that his army was captured. Now it is certain that no general to-day would act in that manner, and if he did it is equally certain that he would be court-martialled. Equally clear is it that comets and eclipses have ceased to infect the modern mind with terror, and are now only objects of study to the learned, and of curiosity to the unlearned. But the difference here is entirely one of knowledge. Our ancestors reacted to the appearance of a comet or an eclipse in a particular manner because their knowledge of these things was of a certain kind. It was not at all a case of feeling, or of degree of feeling, or of having a better brain, but simply a matter of reacting to an environmental influence in terms of an understanding of certain things. Had we the same conception of these things that our ancestors had we should react in the same manner. We act differently because our understanding of things is different. We may put it briefly that the kind of reaction which we make to the things around us is mainly determined by our knowledge concerning their nature.