 The senses are not wholly immaterial faculties in the way that the intellect is, but the respect in which Aquinas thinks they are material is a matter of some dispute. He remarks that the senses operate “through a corporeal organ but not through any corporeal quality” (ST 1a 78.1c). By this he intends a contrast with ordinary physical causation, as when a fire, through the quality of heat, brings water to a boil. In contrast, when we perceive heat, even though the skin is heated in the same way that water is, still the perception itself is not the skin’s being made hot. Rather, “the senses and intellect receive the forms of things spiritually and immaterially, in virtue of a kind of intentional existence” (Comm. de sensu 18.208–10). In general, Aquinas treats the ability to receive forms intentionally as a mark of the mental: