Synaesthesia challenges the classic view of perception, first formulated by Aristotle, that each of the five senses–sight, sound, hearing, taste, and touch–has a distinct and proper sphere of activity (Gage 348). Derived from the Greek syn (meaning union) and aesthesis (sensation), the term synaesthesia is used to describe the “production, from a sense impression of one kind, of an associated mental image of a sense impression of another kind” as well as “the production of a synaesthetic effect” through the “use of metaphor” (OED). The first definition refers primarily to a neuropsychological phenomenon, and is used to describe the experiences of those who hear colors, taste shapes, or otherwise demonstrate the capacity to experience two sensations simultaneously as the result of exposure to a single stimulus (Gage 348). The latter definition describes attempts made to simulate this experience through the use of aesthetic techniques, such as metaphor. Both types of synaesthetic experience raise questions, articulated in W.J.T. Mitchell’s essay “Word and Image,” about the ‘natural’ semiotic and aesthetic order…The nature of the senses, the media, the forms of art is put into question: natural for whom? since when? and why?” The term synaesthesia works both to describe the abnormal perceptions of a small group of people, and to reveal what ‘normal’ perceptions are thought to be.
On Baudelaire’s theory, every colour, sound, odour conceptualized emotion (love, hate, cf affect), every visual image, even if complex (a ship, a carcass), is in some way bound up with an equivalent in each of the other fields: one only, we may infer. (207) While the system works as synaesthesia on the level of metaphor, it is not necessarily a manifestation of the rare neuropsychological disorder. Baudelaire’s system depends on associations evoked by the content of a word. For example, in the poem Correspondences, he writes, “Il est des parfums frais comme des chairs d’enfants”: there are perfumes as fresh as children’s flesh (quoted in Baron-Cohen 99). On the other hand, because Rimbaud sensory connections are revealed in the experience of saying a word, he comes much closer to expressing or simulating the synaesthete’s experience. In Les Voyelles , the sounds made in the pronunciation of the each of the vowels in seems to evoke the color it is paired with (Gage 349). Despite their different approaches to synaesthetic experience, both Baudelaire and Rimbaud seized upon it as a means of innovation. Both wished to change fundamentally the way that people read, understood, and experienced poetry.
Of numerous literary examples, the Symbolist poet Charles Baudelaire often invoked “a unity of sensation” in his texts and Harrison and Barron-Cohen believe this to reflect the poet’s belief that there existed a “natural correspondence between the senses.”  For example, In Correspondences, he includes the lines:
Like long echoes that mingle from far away
In profound and shadowy unity
Vast like the night and like clarity
Scents, sounds and colours respond to one another