Double Brain, Double Person?

M. Thornton, British Journal for the Philosophy of Science, 1994, 45, 761–763.

Excerpt: In “Mind With a Double Brain” (1993), Roland Puccetti skillfully interweaves two distinct themes: (1) a solution to Descartes’ problem as to how and where “the two images which come to us by the two eyes” (1649/1955) are united together, and (2) a proposal that each of us with an intact brain is in fact two person.

Descartes argued that “there must somewhere be a place where the two images which come to us by the two eyes, where the two other impressions which proceed from a single object by means of the double organs of the other senses, can unite before arriving at the soul, in order that they may not represent to it two objects instead of one” (1649/1955, p. 346). Must there be such “a place,” whether in the pineal gland or elsewhere? Puccetti answers No: and I think he is quite correct. The notion that there must be one place in the brain where the “images” and “impressions” from our double organs of sense coalesce is the product of Descartes’ homuncular idea of the mind, the idea that the mind is a unitary indivisible little person at hte crossroads of the brain.

Once we scrap this idea (as I think we must, though I shall not argue the point here), we can appreciate the force of Puccetti’s solution. Instead of a single place, “the full visual field is doubly represented in the double brain” (p. 688). Each brain, i.e. each hemisphere, functions on its own to represent the whole visual field. It is not the case that each brain processes one half of the perceptual input and somewhere in the center of the brain (the pineal gland? the corpus callosum?) the two halves are united.

Puccetti’s solution to Descartes’ problem does not, however, logically committ us to the thesis that each of us is really two persons, each armed with its own brain. For that we need to know more than that each brain can function autonomously; we need to know why a second person should be postulated. My purpose in this paper is to show that, although Puccetti’s two themes do indeed cohere harmoniously together, his argument on the second theme is a non sequitur. For this purpose I shall accept the conceptual framework which Puccetti uses. I shall refer to each hemisphere as “a brain.”; I shall mean by “mind” a locus of mental activities and capabilities (without any theoretical commitment beyond this); and I shall mean by “person” the subject of mental activitie, the possessor of mental capabilities, etc. …