1. The Interaction of the Soul and Body
There are three opinions and traditions, which are hypotheses, concerning the Interaction of the Soul and Body, or the operation of the one upon the other, and of the one together with the other: the first is called Physical Influx, the second Spiritual Influx, and the third Pre-established Harmony.
The first, which is called physical influx, arises from the appearances of the senses, and the fallacies thence derived. For it appears as if the objects of sight, which affect the eyes, flow into the thought and produce it; in like manner speech, which affects the ears, appears to flow into the mind, and to produce ideas there; and it is similar with respect to the senses of smell, taste, and touch. Since the organs of these senses first receive the impressions that flow into them from the world, and the mind appears to think, and also to will, according as these organs are affected, therefore, the ancient philosophers and Schoolmen believed that influx was derived from them into the soul, and hence adopted the hypothesis of Physical or Natural Influx.
 The second hypothesis, which is called spiritual, and by some occasional influx, originates in order and its laws. For the soul is a spiritual substance, and therefore purer, prior, and interior; but the body is material, and therefore grosser, posterior, and exterior; and it is according to order that the purer should flow into the grosser, the prior into the posterior, and the interior into the exterior, thus what is spiritual into what is material, and not the contrary. Consequently, it is according to order for the thinking mind to flow into the sight according to the state induced on the eyes by the objects before them, which state that mind also disposes at its pleasure; and likewise for the perceptive mind to flow into the hearing, according to the state induced upon the ears by speech.
 The third hypothesis, which is called pre-established harmony, arises from the appearances and fallacies of the reasoning faculty; since the mind, in the very act of operating, acts together with and at the same time as the body. Still, every operation is first successive and afterwards simultaneous, and successive operation is Influx, and simultaneous operation is Harmony; as, for instance, when the mind thinks and afterwards speaks, or when it wills and afterwards acts: hence it is a fallacy of the reasoning faculty to establish that which is simultaneous, and to exclude that which is successive.
No fourth opinion concerning the Interaction of the Soul and the Body can be framed in addition to these three; for either the soul must operate upon the body, or the body upon the soul, or both uninterruptedly at the same time.