To casual observers the history of science goes something like this: Greek philosophers introduced the world to rational, naturalistic ways of thinking which freed us from superstition and myth. Sadly, the Roman Empire crumbled, Christianity replaced paganism, religious dogma replaced rationalism, and progress stagnated until about the 16th century when the foundations of science began taking shape. Of course, the real story is more complicated (interested readers should see David Lindberg's The Beginnings of Western Science). At the risk of disorienting casual observers, I am going to explore one of those interesting complications: Medieval neuroscience.
The 12th and 13th centuries witnessed a flourishing of natural philosophy in Christian Europe. While creation, the cosmos, miracles and the nature of God were uppermost on the agenda, medieval natural philosophy also included the biological basis of the human mind. The major brain theory of the time was called the theory of the "inner (or interior) senses," the roots of which ran back to Aristotle (see Simon Kemp's book Cognitive Psychology in the Middle Ages, chapter 4). In his De Anima, Aristotle identified a number of intellectual functions including sensation, imagination and memory. Originally, Aristotle located these functions in the heart, but the renowned Roman physician Galen relocated them to the brain. Physicians after Galen (precisely who is unclear) put these function specifically in the ventricles of the brain given that the ventricles were highly interconnected via nerve fibers to sensory and motor systems throughout the body. Animal spirits flowing from the ventricles through the nerve fibers could then account for the direction of thought and action throughout the body.
Early Christian authorities such as Augustine and Nemesius further elaborated on the inner senses theory by tying specific intellectual functions to particular ventricles. By the 13th century numerous versions of the theory had emerged, but Thomas Aquinas seems to have largely adopted that of the Arab philosopher Avicenna (although not without some modifications).
In this version, there are three ventricles lined up sequentially from front to back. The first ventricle deals with sensory processing and imagination. So inputs from the eyes are sent to the first ventricle where an image is constructed and held. The second ventricle houses the faculties of association and estimation. It is here that two images can be linked together and judgments based on that linkage can be made. For example, a dog might associate his master's face with food and affection, and therefore "decide" to approach the master when fearful. Finally, the last ventricle is the memory store where past associations can be held long-term for later retrieval.
Of course, in its details, this theory is all wrong. The ventricles have nothing to do with cognitive processing. But the critical point is that Aquinas and other Christian scholastics were quite convinced that the brain was essential to cognition. Of even greater interest was why they were convinced of this and the implications it held.
What compelled Medieval scholars to investigate brain function? First, they understood that animals possessed some mental functions such as associative learning and memory. Augustine commented (Literal Meaning of Genesis 3.8.12) on how fish at the fountain of Bella Regius had learned to swim up to passersby in hopes of getting fed (demonstrating both associative learning and memory). If animals shared some cognitive functions with humans then those functions could not be housed in man's immortal rational soul; instead they must be in physical structures shared by both humans and animals. Second, the Medievals were aware that damage to the brain affected cognitive functioning and that damage could often be quite specific -- affecting some cognitive abilities but not others. Nemesius, for example, discusses a case (which he attributes to Galen) of a man suffering from a brain inflammation whose senses and memory were intact, but whose reason and self-control were compromised (he put on a grand display tossing glass vessels out a weaver's shop but correctly naming each vessels as it was thrown).
This theory also presented a theological challenge because it placed important intellectual functions in perishable physical structures. For example, consider the parable of the poor man Lazarus and the rich man who showed him no compassion (Lk 16:19-31). Both die; and while Lazarus is comforted at the bosom of Abraham, the rich man suffers in hell (presumably). The parable assumes that they both have their senses intact. The rich man, in fact, asks that Lazarus be allowed to dip his finger in water and use it to cool his (the rich man's) tongue. The parable also assumes their memories are fully functioning. Abraham specifically instructs the rich man to remember that in life he had comforts while Lazarus suffered. Divine justice requires that the immortal soul be aware of its past deeds and be able to feel reward or punishment. But how is this possible if sensation and memory are housed in the brain -- a physical structure which will obviously cease functioning after death?
Aquinas confronts the problem directly in Summa Theologica (P1 Q89 A6), but he can give no definitive answer. Yes, the soul separated from the body still remembers and knows, but not in the same way as when it was united with the body. Aquinas' discussion is so brief and matter-of- fact that its importance is easily lost. Here's the most formidable thinker of the High Middle Ages confronting one of the great theological challenges of his time (and ours) and his answer is neither novel nor brilliant. It is, however, honest. He simply says, "I don't know."
Science challenges some traditional religious beliefs. But this is nothing new. Thoughtful people have been wrestling with those challenges for centuries. What may be new is the arrogant self-assuredness that masquerades as wisdom. At a time when increasing numbers of both religious people and atheists are so cocksure they've found the Truth, Aquinas provides a refreshing lesson in intellectual humility. He reminds us that sometimes the wisest move one can make is to admit that you don't know everything.