Perceptual Error and Common Sensibles

Do the senses ever err in the act of perceiving? Are all errors of thought errors of judgment only? Can we have certitude given the fallibility of our perceptions and judgments? According to Kenneth Gallagher, "[T]he senses do not err at all...The possibility of error only arises with judgment."¹ In this statement, Gallagher reflects the scholastic understanding of perception that the senses are merely receptors of data and cannot, themselves, be in error. It is only when we form judgments on the basis of these data that we might err.

Of course, we can all understand this concern. We have all erred in some judgment about the data of our senses. Perhaps we thought the road ahead was covered in water when, in fact, it was only a mirage caused by heat rising from the road's surface. The data of our senses were not wrong, but our judgment was. So, it seems at first glance that this problem is correctly sorted out and the answer is to our question is no.

But, as we read on, Gallagher brings to our attention the fact that while we are right to look first at judgment as the place where errors occur, there is a second important consideration, namely, the reliability of our senses. So long as our senses are functioning reliably (see reliabilism), we can in fact limit our discussion of errors of perception to judgment. But, what if a mistake in perception lay not with our judgment of sense data but with the data themselves? Then we could certainly speak of errors of perception beyond errors of judgment and the answer to our question would be yes.

Gallagher notes that Scholastics were aware of this possibility and recognized that "certain requirements have to be met before we can rely on our sense experience to give trustworthy testimony."²  Gallagher explains that sensation "appears to involve a stimulation of a bodily organ, by a physical object, through a medium of action."³ Consequently, the following conditions must obtain for the data of our senses to be trustworthy in perception:⁴

1. The object must be properly proportioned to our kind of senses. These objects must be the kind that we can perceive, whether a wavelength of light or sound, or an object within range of human sight. To have a reliable perception of the object, we must be able to properly sense it.

2. The organ of sense must be a normal and healthy one. If my eyes, ears, hands, etc. are not functioning normally, they will not give me trustworthy data. In this case, my judgments will probably be wrong.

3. Since the object is perceived through a medium, the proper medium for this perception must be present. If I am attempting to properly perceive a painting, I must see it in the right light. Darkness or a red light, would obviously lead to an error in my perception of the painting.

Finally, Gallagher notes that the likelihood of an error in perception goes up in the case of common sensibles, those objects of perception which require the use of more than one sense. The classic example of the bent reed in the water is just this kind of case. To properly sense the reed and to avoid errors in perception, we must not only see the reed, but also feel the reed--gather data regarding its shape and extension. Were we to have these data, we would not fall into the error of judging the reed as bent. In this case, the senses are functioning properly, but our set of data is incomplete.

So, to answer our question, yes we can speak of an error of perception that is not an error of judgment if we speak of an unreliable act of perception on the basis of the conditions listed above. But, when these conditions are satisfied, we are right to look at judgment as the culprit in an erroneous perception. As we have learned, however, an error in judgment in one perception does not mean that we can never have true perception or certitude concerning our true perceptions.

1. 1 Kenneth T. Gallagher, The Philosophy of Knowledge (New York: Sheed & Ward, 1964), 104.

2. Ibid.

3. Ibid.

4. Ibid.