The fourth step in conception is that of classification or generalization, whereby we place individual things in a mental bundle or class, and then this bundle in company with other bundles into a higher class, and so on. Thus we group all the individual small birds having certain characteristics into a species, then several related species into a larger family, and this into a still larger, until finally we group all the bird families into the great family which we call "birds" and of which the simple term "bird" expresses the general concept. Jevons says: "We classify things together whenever we observe that they are like each other in any respect, and therefore think of them together. In classifying a collection of objects, we do not merely put together into groups those which resemble each other, but we also divide each class into smaller ones in which the resemblance is more complete. Thus the class of _white substances_ may be divided into those which are solid, and those which are fluid, so that we get the two minor classes of solid-white and fluid-white substances. It is desirable to have names by which to show that one class is contained in another, and, accordingly, we call the class which is divided into two or more smaller ones the _genus_, and the smaller ones into which it is divided, the _species_." Every _species_ is a small family of the individuals composing it, and at the same time is an individual species of the genus just above it; the _genus_, in turn, is a family of several species, and at the same time an individual genus in the greater family or genus above it. The student may familiarize himself with the idea of generalization by considering himself as an individual, John Smith. John represents that unit of generalization. The next step is to combine John with the other Smiths of his immediate family. Then this family may be grouped with his near blood relations, and so on, until finally all the related Smiths, near and remote, are grouped together in a great Smith family.