The Senses: A Spiritual Power

An excerpt from my new book, Affective Knowledge in the Theology of Thomas Aquinas

In the opening chapters of the commentary on Aristotle’s Metaphysics, Aquinas makes clear that the ground of all human knowing is sensation, which mediates the external world to our  intellects. I see a four-legged creature. Its lugubrious brown eyes stare intently up at me. Its tail wags furiously. An elongated tongue dripping saliva dilates from its mouth. Such a portrait can only be apprehended by my senses. To answer the question “What is that?” the senses have primacy. (Of course, as we will discuss shortly, the senses alone are incapable of answering the question “What is that?”)

The senses are material and passive powers; that is to say, they are bodily organs that receive sense data. Aquinas follows Aristotle in maintaining that each sense has a proper “sensible” to which it responds. (For example, the proper “sensible” of sight is color.) Because the sense organ is material and passive, it stands in potency to receive the form (or species) of the sensible reality to which it is then conformed. Recall that for classical hylomorphism the form is the active principle impressing itself on matter, the passive principle. It is form that renders intelligibility, structure, coherence; in short, form “shapes” matter, configuring it to itself.

If the sense organ is material and passive, we might ask why our senses are not materially conformed to the species perceived. When my eyes receive the sensible species of a Labrador Retriever described above, my eyes do not become a dog. Why not? Aquinas distinguishes  between two modes in which a species can be received: natural and spiritual.

Now, change is of two kinds, one natural, the other spiritual. Natural change takes place by the form of the agent being received according to its natural being, into the thing changed, as heat is received into the thing heated. Whereas spiritual change takes place by the form of the agent being received, according to a spiritual mode of being, into the thing changed, as the form of color is received into the pupil which does not thereby become colored. Now, for the operation of the senses, a spiritual change is required, whereby an intention of the sensible form comes to be in the sensile organ. Otherwise, if a natural change alone sufficed for the sense’s action, all natural bodies would feel when they undergo alteration. (ST I q. 78 a. 3.)

The “natural” (or material) reception of a species entails that the form impresses itself on the matter and configures the matter according to that form. Thus, the matter of water can receive the natural species of heat. If sensation operated according to such a natural reception of form, then the matter of the sense organ—say the eye—would be materially conformed to that which it sees: my eye would be transformed into a Labrador Retriever the moment I see the dog. However, the “spiritual” (or intentional) reception of form entails that a form can also be received without matter. As such, for Aquinas, species can exist in two distinct modes: “The form has another way of being in the sense from that which it has in the object sensed. For in the thing being sensed it has a natural being (esse naturale), but in the sense it has intentional and spiritual being (esse intentionale et spirituale).” In one way the species configures the matter of the extramental reality; in another way it configures the spiritual reality of the knowing subject.

Sensation entails that the senses receive a likeness (similitudo) of the extramental reality. This “sensible species” impressed on the sense organ is the necessary starting point for all human cognition. As Stump explains, “In cognition the similitude is based on spiritual reception in the cognizer of the form of the thing cognized. A cognizer and the object of his cognition share a  form, but the similitude in this case is a representation.” In short, sensation entails the transference of the species of sensible reality into the realm of spirit.