The reason of the difference in the concepts of several persons is that very few of our concepts are nearly perfect; the majority of them are quite imperfect and incomplete. Jevons gives us an idea of this in his remarks on classification: "Things may seem to be very much like each other which are not so. Whales, porpoises, seals, and several other animals live in the sea exactly like a fish; they have a similar shape and are usually classed among fish. People are said to go whale fishing. Yet these animals are not really fish at all, but are much more like dogs and horses and other quadrupeds than they are like fish. They cannot live entirely under water and breathe the air contained in the water like fish, but they have to come to the surface at intervals to take breath. Similarly, we must not class bats with birds because they fly about, although they have what would be called wings; these wings are not like those of birds, and, in truth, bats are much more like rats and mice than they are like birds. Botanists used at one time to classify plants according to their size, as trees, shrubs, or herbs, but we now know that a great tree is often more similar in character to a tiny herb than it is to other great trees. A daisy has little resemblance to a great Scotch thistle; yet the botanist regards them as very similar. The lofty growing bamboo is a kind of grass, and the sugar cane also belongs to the same class with wheat and oats." It is a matter of importance that clear concepts should be formed regarding at least the familiar things of life. The list of clear concepts should be added to from time to time by study, investigation, and examination. The dictionary should be consulted frequently, and a term studied until one has a clear meaning of the concept the term seeks to express. A good encyclopedia (not necessarily an expensive one, in these days of cheap editions) will also prove very useful in this respect. As Halleck says: "It must be borne in mind that most of our concepts are subject to change during our entire life; that at first they are made only in a tentative way; that experience may show us, at any time, that they have been erroneously formed, that we have abstracted too little or too much, made the class too wide or too narrow, or that here a quality must be added or there one taken away."