The human senses have traditionally numbered five, dating from Aristotle’s On the Soul, Greek Περὶ Ψυχῆς (Perì Psūchês), Latin De Anima. An English translation with commentary by St. Thomas Aquinas is here. Modern science has identified many more senses (in humans and other animal species) depending on definition (see here). In Metaphysics Aristotle said “There is nothing in the intellect that was not previously in the senses”. The human senses considered by Aristotle in De Anima were (in order): Sight, Hearing, Smell, Taste, Touch. It is probably not a coincidence that this order corresponds to the maximum distance over which these senses have command. Subsequent Aristotelian thinkers (such as Aquinas) seem to take this order as indicating a spirituality/materiality continuum, from sight (most spiritual/heavenly) to touch (most material/earthly) as in the Summa Theologica Question 83 here. Aquinas says that “Further, of all the senses the sight is the most spiritual and the nearest to reason” (Article 4, Objection 3) and also that touch is the sense most infected by original sin (Article 4, Reply to Objection 3). It might not be an exaggeration to maintain that in contemporary Western culture the sense of sight is the most highly privileged and touch the most underestimated even today.
Here is a quote from Denis Diderot (1713-1784) with a different take on the senses:
“And I found that of all the senses the eye was the most superficial, the ear the most haughty, smell the most voluptuous, taste the most superstitious and inconstant, touch the most profound and philosophical” Lettre sur les aveugles à l’usage de ceux qui voient (Letter on the Blind, for the Usage of those who can See)
In his “Phenomenology of Perception”, Merleau-Ponty promotes the idea of the “unity of the senses”.
After discussing a certain unusual case involving brain injury which caused a disconnect in the subject’s tactile and visual experience of the world, M-P draws the implication that
“…there is not in the normal subject a tactile experience and also a visual one, but an integrated experience to which it is impossible to gauge the contribution of each sense.” (PP137)
Some more quotes:
“Let us therefore say rather, borrowing a term from other works, that the life of consciousness—cognitive life, the life of desire or perpetual life—is subtended by an ‘intentional arc’ which projects round about us our past, our future, our human setting, our physical, ideological and moral situation, or rather which results in our being situated in all these respects. It is this intentional arc which brings about the unity of the senses, of intelligence, of sensibility and motility.” (PP157)
“… my body is not a collection of adjacent organs, but a synergic system, all the functions of which are exercised, and linked together in the general action of being in the world, in so far as it is the congealed face of existence.” (PP272)
“It is my body which gives significance not only to the natural object, but also to cultural objects like words…..The word ‘hard’ produces a sort of stiffening of the back and neck, and only in a secondary way does it project itself into the visual or auditory field and assume the appearance of a sign or word. Before becoming the indication of a concept it is first of all an event which grips my body, and this grip circumscribes the area of significance to which it has reference.” (PP273)
“Movement, understood not as objective movement and transference in space, but as a project towards movement or ‘potential movement’ forms the basis for the unity of the senses.” (PP272)
“Consciousness is in the first place not a matter of ‘I think’ but of ‘I can’.” (PP159, attributed by M-P to Husserl)
Another way of approaching the idea of “unity of the senses” might be through consideration of the “abnormal” syndrome called synaesthesia. “Synaesthesia is experienced when stimulation of one sensory modality gives rise to a perception in a second modality, without that second modality having received any direct stimulation.” (“Synaesthesia”, Encyclopedia of Cognitive Science, John Wiley and Sons, 2006). For example, some synaesthetes can see colours and simple shapes in sounds. Usually two senses are involved, rarely three or more. The perceptions in one sense triggered by a stimulus in a second sense are usually very consistent within one individual, but not necessarily between synaesthetes. A recent theory which has gained a lot of attention, and for which there is some experimental evidence is the Neonatal Synaesthesia Hypothesis. This theory proposes that there is a normal phase of synaesthesia in development, that all babies experience sensory input in an undifferentiated way in early infancy, probably up to about 4 months in age, and that for some people (adult synaesthetes) the normal dying off or inhibition of the connections between different sensory modalities in infancy has not occurred. Interestingly, while modern cognitive scientists might assume the idea of a developmental pathway from some form of synaesthesia to separate sense modalities to be quite recent, in fact it has a long history, including literary works such as Jean-Jacques Rousseau’s Emile (1762) and Mary Shelley’s Frankenstein (1818), as well as scientific works such as William James’ Principles of Psychology (1890). See here for details.
A book that looks interesting enough to try to track down is Les Cinq Sens (The Five Senses) by French philosopher Michel Serres. There is a review here. Serres does not seem to have a taste for the phenomenology of Merleau-Ponty. Or it may rather be that he considers him too dry, academic and bloodless. From the review: “Les Cinq Sens is part of the turn which Serres’s work undertook during the 1980s from a certain kind of philosophically respectable and recognisable commentary to the work of invention, a work characterised by lightness, freedom, associativeness, caprice.”