3. Exceptional and rare cases. Flournoy, in his important work on “coloured hearing,” rightly explains this anomaly by “emotional association.” We know that several hypotheses on the origin and cause of this phenomenon have been constructed. On the embryological one, it would be the result of an incomplete differentiation between the sense of sight and that of hearing; a survival, we are told, from a primitive epoch when this state was the rule. On the anatomical theory, we suppose anastomoses between the cerebral centres of the visual and auditory sensations. Besides these we have the physiological theory, or that of nervous irradiation, and the psychological, or that of association. I do not inquire if all cases may be reduced to a single explanation; certainly most seem reducible to association. We are not, however, dealing with any and every form of association—it must be a psychological one, as Flournoy was the first to remark. “By emotional association, I mean that which establishes itself between two impressions, not on account of a qualitative resemblance (for the two may be as disparate as sound and colour), nor in virtue of their regular and frequent concurrence in the consciousness, but in consequence of the analogy between their emotional characteristics. Each sensation or perception possesses, in fact, along with its objective quality or its intellectual content, a sort of subjective coefficient, springing from the roots which it sends down into our being, and from the peculiar way in which it impresses, pleases or displeases, excites or subdues us, in a word, makes our whole nature vibrate. We can conceive how two absolutely heterogeneous sensations, incommensurable as far as their objective content is concerned, such as a colour and the sound _i_, may be comparable with one another and resemble each other more or less, by virtue of vibrations produced by them in the organism; and by the same process of thought it is conceivable that this emotional factor might become a link between the two, an associative bond by means of which one awakens the other.”[119] ----- Footnote 119: Th. Flournoy, _Des phénomènes de Synopsie_ (1893), p. 20. ----- Let us add that we meet, though much more rarely, with cases of coloured smell and taste, and even, it appears, of coloured pain.[120] This abnormal association between determinate colours and determinate tastes, odours, pains, may be explained in the same manner. ----- Footnote 120: Suarez de Mendoza, _L’audition colorée_ (1890), pp. 58, 59. -----