One and the same quality belongs to many phenomena. There is a whole series of things which are blue, round, and smooth. The possession of a common quality is a condition of similarity, which is greater in proportion to the number of common qualities. Every single quality, however, belongs to a habitually associated group of qualities, and can by the mechanism of simultaneity arouse the memory-image of this group. In consequence of their similarity, therefore, the memory-images can be aroused of all those groups, which resemble each other in some quality. The colour blue is a quality which belongs equally to the cheerful sky, the cornflower, the sea, certain eyes, and many military uniforms. The perception of blue will awaken the memory of some or many blue things which are only related through their common colour. Similarity is therefore another cause of the association of ideas. It is a distinctive characteristic of the brain-cell to elaborate at the same time both a presentation and its opposite. It is probable that what we perceive as its opposite is generally, in its original and simplest form, only the consciousness of the cessation of a certain presentation. As the fatigue of the optic nerve by a colour arouses the sensation of the complimentary colour, so, on the exhaustion of a brain-cell through the elaboration of a presentation, the contrary presentation appears in consciousness. Now, whether this interpretation be right or not, the fact itself is established through the ‘contradictory double meaning of primitive roots,’ discovered by K. Abel.[72] Contrast is the third cause of the association of ideas. Many phenomena present themselves in the same place close to, or after, one another; and we associate there, presentation of the particular place with those objects, to which it is used to serve as a frame. Simultaneity, similarity, contrast, and occurrence in the same place (contiguity), are thus, according to Wundt, the four conditions under which phenomena will be connected in our consciousness through the association of ideas. To these James Sully[73] believes yet a fifth should be added: presentations which are rooted in the same emotion. Nevertheless all the examples cited by the distinguished English psychologist demonstrate without effort the action of one or more of Wundt’s laws.