INTRODUCTION
All thought begins with conceptions to a certain extent generalised, and thence is developed in two directions. On the one hand, generalisations become wider and wider, binding together by common properties a larger and larger number of phenomena, and so embracing a wider field of the world of facts. On the other hand, thought approaches more closely the meeting-point of all conceptions, the individual, the concrete complex unit towards which we approach only by thinking in an ever-narrowing circle, and by continually being able to add new specific and differentiating attributes to the general idea, “thing,” or “something.” It was known that fishes formed a class of the animal kingdom distinct from mammals, birds, or invertebrates, long before it was recognised on the one hand that fishes might be bony or cartilaginous, or on the other that fishes, birds and mammals composed a group differing from the invertebrates by many common characters.
The self-assertion of the mind over the world of facts in all its complexity of innumerable resemblances and differences has been compared with the rule of the struggle for existence among living beings. Our conceptions stand between us and reality. It is only step by step that we can control them. As in the case of a madman, we may first have to throw a net over the whole body so that some limit may be set to his struggles; and only after the whole has been thus secured, is it possible to attend to the proper restraint of each limb.
Two general conceptions have come down to us from primitive mankind, and from the earliest times have held our mental processes in their leash. Many a time these
