Having formed general ideas, or Concepts, it is important that we associate them with other general ideas. In order to fully _understand_ a general idea we must know its associations and relations. The greater the known associations or relations of an idea, the greater is our degree of understanding of that idea. If we simply know many thousands of separated general ideas, without also knowing their associations and relations, we are in almost as difficult a position as if we merely knew thousands of individual percepts without being able to classify them in general concepts. It is necessary to develop the faculty of associating ideas into groups, according to their relations, just as we group particular ideas in classes. The difference, however, is that these group-ideas do not form classes of a genus, but depend solely upon associations of several kinds, as we shall see in a moment. Halleck says: "All ideas have certain definite associations with other ideas, and they come up in groups. There is always an association between our ideas, although there are cases when we cannot trace it.... Even when we find no association between our ideas, we may be sure that it exists.... An idea, then, never appears in consciousness unless there is a definite reason why this idea should appear in preference to others." Brooks says: "One idea or feeling in the mind calls up some other idea or feeling with which it is in some way related. Our ideas seem, as it were, to be tied together by the invisible thread of association, so that as one comes out of unconsciousness, it draws another with it. Thoughts seem to exist somewhat in clusters like the grapes of a bunch, so that in bringing out one, we bring the entire cluster with it. The law of association is thus the tie, the thread, the golden link by which our thoughts are united in an act of reproduction." The majority of writers confine their consideration of Association of Ideas to its relation to Memory. It is true that the Laws of Association play an important part in Memory Culture, but Association of Ideas also form an important part of the general subject of Thought-Culture, and especially in the phase of the latter devoted to the development of the Understanding. The best authorities agree upon this idea and state it positively. Ribot says: "The most fundamental law which regulates psychological phenomena is the Law of Association. In its comprehensive character it is comparable to the law of attraction in the physical world." Mill says: "That which the law of gravitation is to astronomy, that which the elementary properties of the tissues are to physiology, the Law of Association of Ideas is to psychology." There are two general principles, or laws, operative in the processes of Association of Ideas, known as (1) Association by Contiguity; and (2) Association by Similarity, respectively.