1. Concerning the interaction between the soul and the body, or the operation of one into the other, and of one with the other, there are three opinions and traditions, which are hypotheses. The first is called Physical Influx, the second Spiritual Influx, and the third Pre-established Harmony.

The first, which is called Physical Influx, is from the appearances of the senses and the fallacies therefrom, since it appears as if the objects of sight, which affect the eyes, flow into thought and produce it; in like manner that speech, which moves the ears, flows into the mind and introduces ideas there; and similarly with the senses of smell, taste, and touch. Since the organs of these senses first receive the impressions from contact with the world, and according as they are affected the mind appears to think and also to will; for this reason the ancient philosophers and scholastics believed that influx was derived from these organs into the soul, and thus they adopted the hypothesis of physical or natural influx.

[2] The second hypothesis, which is called Spiritual Influx, by some occasional influx, is from order and its laws; since 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 purer should flow into grosser, prior into posterior, and interior into exterior, thus spiritual into material, and not the reverse. Consequently it is of order that the thinking mind should flow into the sight according to the state induced on the eyes from objects, which state the mind also disposes at will; and likewise the perceptive mind into the hearing according to the state induced on the ears from speech.

[3] The third hypothesis, which is called Pre-established Harmony, is from the appearances and fallacies of reason, since the mind in its operation acts together and at the same time with the body. But yet every operation is first successive and afterward simultaneous. Successive operation is influx, and simultaneous operation is harmony; as when the mind thinks and afterward speaks, or when it wills and afterward acts. It is therefore a fallacy of reason to establish simultaneous operation and to exclude successive. Besides these three opinions concerning the interaction of the soul and the body, no fourth is possible, for either the soul must operate on the body, or the body on the soul, or both continually together.