The relation between the external world and the organism is originally very simple. Movements are continually occurring in nature, and the protoplasm of living cells perceives these movements. Unity of effect corresponds to unity of cause. The lowest animals perceive of the outer world only this, that something in it changes, and possibly, also, whether this change is marked or slight, sudden or slow. They receive sensations differing quantitatively, but not qualitatively. We know, for example, that the proboscis, or syphon, of the _Pholas dactylus_, which contracts more or less vigorously and quickly at every excitation, is sensitive to all external impressions--light, noise, touch, smell, etc. This mollusc sees, hears, feels and smells, therefore, with this simple organ; his proboscis is to him at once eye, ear, nose, finger, etc. In the higher animals the protoplasm is differentiated. Nerves, ganglia, brain and sense-apparatus are formed. The movements of nature are now perceived in a variety of ways. The differentiated senses transform the unity of the phenomenon into the diversity of the percept. But even in the highest and most differentiated brain there still remains something like a very distant and very dim remembrance that the cause which excites the different senses is one and the same movement, and there are formed presentations and conceptions which would be unintelligible if we could not concede this vague intuition of the fundamental unity of essence in all perceptions. We speak of ‘high’ and ‘deep’ tones, and thus give to sound-waves a relationship in space which they cannot have. In the same way we speak of tone-colour, and, conversely, of colour-tones, and thus confound the acoustic and optic properties of the phenomena. ‘Hard’ and ‘soft’ lines or tones, ‘sweet’ voices, are frequent modes of expression, which depend on a transference of the perception of one sense to the impressions of another. In many cases this method of speech may no doubt be traced to mental inertia. It is more convenient to designate a sense-perception by a word which is familiar, though borrowed from the province of another sense, than to create a special word for the particular percept. But even this loan for convenience’ sake is possible and intelligible only if we admit that the mind perceives certain resemblances between the impressions of the different senses--resemblances which, although they are often to be explained by conscious or unconscious association of ideas, are oftener quite inexplicable objectively. It only remains for us to assume that consciousness, in its deepest substrata, neglects the differentiation of phenomena by the various senses, passes over this perfection attained very late in organic evolution, and treats impressions only as undifferentiated material for the acquirement of knowledge of the external world without reference to their origin by way of this or that sense. It thus becomes intelligible that the mind mingles the perceptions attained through the different senses, and transforms them one into another. Binet[150] has established, in his excellent essays, this transposition of the senses in hysterical persons. A female patient, whose skin was perfectly insensible on one half of her body, took no notice when, unseen by herself, she was pricked with a needle. But at the moment of puncture there arose in her consciousness the image of a black (in the case of another invalid, of a bright) point. Consciousness thus transposed an impression of the nerves of the skin, which, as such, was not perceived, into an impression of the retina, of the optic nerve.