Once the relations of qualities have been observed and formulated, however, we need no longer attend to any part of the fact at all. Instead of the actual qualities we now take symbols, words, for example, or letters, or other signs, and with these symbols we make for ourselves diagrams of the relations in which we have observed that the qualities which they represent have stood to each other. Thus we might use the words “lightning before thunder” or first an L and then a T, to express the fact that in a storm we usually observe the quality of flashing before the quality of rumbling. Such laws do not actually reveal new facts to us, they can only tell us, provided we actually know a fact belonging to a given class, to what other class facts which we shall know bye and bye will belong. Thus, once we have classified facts as belonging to two classes, daylight and darkness, and have observed the invariable alternation of facts belonging to these classes, then, whenever we know directly facts which can be classed as daylight, we can predict, according to our law of the alternation of the two classes, that bye and bye these facts will give place to others which can be classed as darkness and that bye and bye these in their turn will be replaced by facts which can again be classed as daylight. The practical value of being able to make even such elementary predictions as these is obviously enormous, and this value increases as applied science, which is built up simply by the formulation of more and more comprehensive general laws of this type, widens the field of facts which can be explained. Once the laws are known, moreover, we are able to say to what class the facts must have belonged which preceded a fact of any given class just as easily as we can say to what class the facts which are to follow it will belong. Thus, given a fact which can be classed as daylight, we can infer, by means of the law of the alternation of the classes daylight and darkness, not only that facts which can be classed as darkness will follow bye and bye, but also that facts of that class must have gone before. In this way we can explain the causes of all classifiable facts equally with their effects and so bridge over the gaps in our direct knowledge by creating a unified plan of the interrelations of all the classes to which facts can belong. By means of this plan we can explain any fact (that is classify its causes and effects), provided we can fit it into one or other of the known classes. This again is of enormous practical use because, when we know to what class present facts must belong if they are to be followed by the class of facts which we want, or not to be followed by those which we do not want, we can arrange our present facts accordingly.