Now, memory is a property not only of the nerve-cell, but also of the nerve-fibre, which is only a modification of the cell. The fibre has a recollection of the stimulus which it conveyed, in the same way as the cell has of that which it has transformed into presentation and motion. A stimulus will be more easily conducted by a fibre which has already conveyed it, than by one which propagates it for the first time from one cell to another. Every stimulus which reaches a cell will take the line of least resistance, and this will be set out for it along those nerve-tracks which it has already traversed. Thus a definite path is formed for the course of a stimulus-wave, a customary line of march; it is always the same nerve-cells which exchange mutually their stimulus-waves. Presentation always awakens the same resulting presentations, and always appears in consciousness accompanied by them. This procedure is called the association of ideas. It is neither volition nor accident that determines to which other cells a disturbed cell habitually communicates its stimulus, which accompanying impressions an aroused presentation draws with it into consciousness. On the contrary, the linking of presentations is dependent upon laws which Wundt especially has well formulated. Those who have not been born blind and deaf (like the unfortunate Laura Bridgman, cited by all psychologists) will never be influenced by one external stimulus only, but invariably by many stimuli at once. Every single phenomenon of the outer world has, as a rule, not only one quality, but many; and since that which we call a quality is the assumed cause of a definite sensation, it results that phenomena appeal at once to several senses, are simultaneously seen, heard, felt, and moreover are seen in different degrees of light and colour, heard in various nuances of timbre, etc. The few phenomena which possess only one quality and arouse therefore only one sense, _e.g._, thunder, which is only heard, although with varying intensity, occur nevertheless in conjunction with other phenomena, such as, to keep to thunder, with a clouded sky, lightning and rain. Our brains are therefore accustomed to receive at once from every phenomenon several stimuli, which proceed partly from the many qualities of the phenomenon itself, and partly from the phenomena usually accompanying it. Now, it is sufficient that only one of these stimuli should reach the brain, in order to call into life, in virtue of the habitual association of the memory-images, the remaining stimuli of the same group as well. Simultaneity of impressions is therefore a cause of the association of ideas.