Remarks

“For this can be true, that sensibles under the aspect of their sensibility do not exist; i.e., if they are considered under the aspect of sensibles actualized, they do not exist apart from the senses, for they are sensibles actualized insofar as they are present in a sense. And according to this every actualized sensible is a certain modification of the subject sensing, although this would be impossible if there were no sensory beings. But that the sensible objects which cause this modification in a sense should not exist is impossible. This becomes clear as follows: when some subsequent thing is removed it does not follow that a prior thing is removed. But the thing producing the modification in a sense is not the perception itself, because a perception is not the perception of itself but of something else, and this must be naturally prior to the perception just as a mover is prior to the thing which is moved. For sight does not see itself but sees color.” (Aquinas, Commentary on Aristotle’s Metaphysics, 4.13.706)

The idea behind Aquinas’s argument (with some added interpretation on my part) is that there exists a correlation between object and subject with respect to the objects of sense (see below).

First, from the side of the subject, there is the sensible as sensed, viz., the redness seen, sweetness tasted, and so on. Such sensations exist as modifications in the subject. Apart from this they have no “in-itself” existence. However, they are not simply subjective properties, but instead have an objective basis. Following Aquinas, we may say that they exist as sensibles actualized in sense. 

Indeed, the occurrence of such modifications indicates causes that cannot be reduced to the subject. The intense heat of a fire or an icy wind can be felt as impinging upon the senses. We experience ourselves as passively undergoing these modifications. We can often do nothing more than suffer (passio) them. Compare the experience of an extremely loud noise (e.g., a jet engine) to that of volitionally raising one’s hand. In the former case, a cause is identified outside or apart from our agency. In the latter cause, we identify our own agency as the cause.

Sensibles point to the second side of the correlation. The sensation of cold indicates a force of nature that exists in the world. Objects themselves are the ultimate source or cause of the modifications that result in sensibles. The modifications correspond to real world qualities. The sensation of cold corresponds to naturally low temperatures, heat to high temperatures. Distinct colors correspond to distinct wavelengths in nature, and so on.

Even if sense qualities as modifications of the subject are distinct in kind from the object qualities as the source or cause these modifications, the two are nonetheless related. Objects in the world cause the modifications of sense, and object qualities correlate to sensed qualities.

Following Aquinas, we may conclude that sense qualities have an objective basis in the qualities of real objects in the world, as the cause of the modifications in the subject that occur during an act of perception.