1. THERE have been three grand theories—hypotheses, actually—of the nature of the interaction between the soul and the body, meaning the way one affects the other and cooperates with it. The first of these is known as “physical inflow,” the second “spiritual inflow,” and the third “preestablished harmony.”
The first, called physical inflow, is based on the way things seem to our senses and on the deceptive appearances that result, since it seems as though the objects of sight that affect our eyes flow into our thinking and make thoughts happen. In the same way, it seems as though the conversations that affect our ears flow into our minds and cause ideas to form there. We could say much the same of smell, taste, and touch as well. Because our sensory organs first receive the stimuli that impinge on them from the world, and because the mind seems both to think and to will things in response to those stimuli, the classical philosophers and Scholastics believed that an inflow from our sense impressions impinged on our soul. This led them to come up with the hypothesis that the inflow between the two was physical, or earthly, in nature.
[2] The second view, called spiritual inflow (some call it “occasional inflow”), is based on the laws of the divine design. It sees the soul as a spiritual substance and therefore something purer, primary, and inward; while the body is matter and is therefore coarser, secondary, and outward. In the divine design, what is purer flows into what is coarser, what is primary flows into what is secondary, and what is inward flows into what is outward. Therefore what is spiritual flows into what is material, and not the reverse. To be more specific, the part of our mind devoted to thinking flows into our eyesight in accordance with the state imposed on our eyes by the objects we are seeing, and also imposes its own priorities on that state. In the same way, the part of our mind devoted to perception flows into our hearing in accordance with the state imposed on our ears by what is being said.
[3] The third view, called preestablished harmony, is based on plausible rational fallacies, because when the mind is producing an effect it does so in unison and simultaneously with the body. However, every deed starts out sequential and only later becomes simultaneous. Inflow is sequential, and harmony is simultaneous, as we can see when the mind thinks something and then says it or when it wills something and then does it. It is a rational fallacy, then, to insist that there is a simultaneous aspect yet deny that there is a [prior] sequential one.
These three are the only possible theories—there is no fourth. The soul activates the body, or the body activates the soul, or the two are constantly acting in unison.
