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Neuroscience and Thomas Aquinas

Recent science has shown the fruitfulness of taking the brain to be the seat of all those mental faculties medieval thinkers, such as Thomas Aquinas, had attributed to the soul. Therefore, we consider here a variety of results from neuroscience which make it appear that the various human capacities once attributed to the soul are better understood as capacities of the human brain.

One sort of research concerns the localizing of various cognitive and affective functions in specific regions or distributed systems of the brain. This research began by studying victims of brain damage, correlating lost faculties with localized damage discovered during autopsies. With the development of CAT scans (computerized axial tomography), it has become possible to study correlations between structural abnormalities and the behavior of people while they are alive. Further, MRI scans (magnetic resonance imaging) now provide quite detailed pictures of the brain, more easily revealing locations of brain damage. And PET scans (positron emission tomography) allow research correlating localized brain activity with the performance of specialized cognitive tasks.

These varied techniques have allowed for the localization of a vast array of cognitive functions. To show the extent to which current science now studies the capacities once attributed to the soul, let us consider in more detail the account developed by Thomas Aquinas of the hierarchically ordered faculties, or powers, of the soul.

Vegetative Faculties

The `lowest' powers of the human soul, shared with plants and animals, are the vegetative faculties of nutrition, growth, and reproduction. All of these processes are now fairly well understood in biological terms, especially since the discovery of DNA. The brain is significantly involved here, in that neurochemicals play a large role in appetite and sex drive; while pituitary hormones control growth.

Sensitive Faculties

Next higher are the sensitive faculties, shared with animals but not plants. They include the exterior senses of sight, hearing, smell, taste, and touch, as well as the four "interior senses," called sensus communis, phantasia (imagination), vis aestimativa, and vis memorativa (memory). The sensus communis is the faculty that distinguishes and collates data from the exterior senses. An example of this faculty would be associating the bark and the brownness of the fur with the same dog. The vis aestimativa allows for apprehensions that go beyond sensory perception. Here, an example would be apprehending the fact that something is useful or useless; friendly or unfriendly. This sensitive level of the soul also provides for the power of locomotion and for lower aspects of appetite -- the ability to be attracted to sensible objects. This appetitive faculty is further subdivided between a simple tendency toward or away what is sensed as good or evil, and a more complex inclination to meet bodily needs or threats with appropriate responses: attack, avoidance, or acquiescence. Together, these appetitive faculties (all still at the sensitive level) provide for eleven kinds of emotion: love, desire, delight, hate, aversion, sorrow, fear, daring, hope, despair, and anger.

Locomotion is now known to be controlled by the motor cortex -- running across the top of the brain -- and by the efferent nervous system.

Great progress has been made in tracing the processes involved in sensation.Churchland, The Engine of Reason, 159 For example, signals are transmitted from two different kinds of light-sensitive cells in the retina, through a series of processors, and on to the visual cortex. Smell involves the sending of signals from six different kinds of receptor cells to the olfactory lobes.

The task Aquinas assigned to the "interior sense" sensus communis -- the ability to synthesize input from the various external senses -- is now studied by neuroscientists as "the binding problem."

The "interior sense" of memory, identified by Aquinas, has also been researched a great deal. Long-term memory is now understood to arise from patterns of connections within the neural network. Short-term memory is believed to be enabled by a system of "recurrent pathways," such that information is processed, recycled, and then fed into the process again. The hippocampus is involved in converting short-term into long-term memory, but how this happens is not yet known.

One of the most interesting findings involves the localization of specific sorts of memory. Paul Churchland presents a map of the brain showing regions involved in language memory, with different locations being responsible for verb access, proper name access, common noun access, and color terms.Churchland, The Engine of Reason, 157 The parietal lobes are an example, as they are involved in our memory of faces.

PET scans make it possible to record localized elevations of neuronal activity. Paul Churchland reports an experiment in which his wife, Patricia, was asked to perform a task involving her visual imagination. The activity in her visual cortex was elevated exactly during the time she was doing the exercise, but not to the same extent as when she received external visual stimulation. Paul Churchland hypothesizes that visual imagination involves the systematic stimulation of the visual cortex "by way of recurrent axonal pathways descending from elsewhere in the brain."Churchland, The Engine of Reason, 179

The vis aestimativa of Aquinas included the ability to distinguish between the friendly and the unfriendly, the useful and the useless. One clear instance of this is our ability to read others' emotions. While there does not seem to be a single location responsible for this capacity, there are patients whose brain damage has resulted in its loss. For instance, Churchland describes the patient "Boswell," who suffers from extensive lesions to the frontal pole of both temporal lobes, and to the underpart of the frontal cortex. One, among many, of his mental deficits is the inability to perceive emotion. Churchland reports:

I watched as Boswell was shown a series of dramatic posters advertising sundry Hollywood movies. He was asked to say what was going on in each. One of them showed a man and a woman, in close portrait, confronting one another angrily. The man's mouth was open in a plainly hostile shout. Boswell, without evident discomfort or dismay, explained that the man appeared to be singing to the woman.Antonio R. Damasio, Descartes' Error: Emotion, Reason, and the Human Brain (New York: G.P. Putnam's Sons, 1994).

The sensitive appetite postulated by Aquinas was responsible for emotions such as desire, delight, sorrow, and despair. Studies of the etiology of mental illnesses involving inappropriate affect have shown a significant role for neurotransmitters such as serotonin.

Rational Faculties

The rational faculties described by Aquinas are distinctively human: passive and active intellect and will. The will is a higher appetitive faculty whose object is the good. Since God is ultimate goodness, this faculty is ultimately directed toward God. The two faculties of the intellect enable abstraction, grasping or comprehending the abstracted universals, judging, and remembering. Morality is a function of attraction to the good, combined with rational judgment in reference to what the good truly consists.

These higher mental faculties Aquinas attributed to the rational soul are further from being understood. However, all of them involve language. Even if we do not understand how these mental faculties depend on brain functioning, we know that they do because of the close association of linguistic abilities with specific brain areas, especially Wernicke's area and Broca's area.


To review, a variety of results make it appear that the various human capacities Aquinas had attributed to the soul are better understood as capacities of the human brain. In fact, these capacities are attributable to specific regions of the brain.

These conclusions are not uncontroversial. First, there is the argument within neuroscience over specialization versus globalism. That is, many would argue that each of the mental capacities listed above is much more a result of global functioning of the brain, not localized functioning. We need not get into this argument; all that needs to be pointed out is that the regions cited above are involved in the specified functions, since all we know is that, if a region is damaged by illness or injury, a corresponding function is lost. Second, there are still some philosophers and scientists who maintain a dualist account of the mind and brain. They point out that however precise science may become in associating mental functions with the brain, science will never prove it is the brain performing the functions. It may simply be the case that functions performed by an independently existing mind, or soul, are just highly correlated with brain functions.

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