[PHI 3000] Sensory Illusions

In the Meditations on First Philosophy, Descartes puts forward something like the following argument:

1. My senses (particularly, my sense of vision) tell me that the line on the right is longer than the line on the left (see image of Muller-Lyer illusion below).
2. Based on what I see, I come to believe that the line on the right is longer than the line on the left.
3. But the two lines are of equal length (as we can find out by measuring them).
4. Hence, my senses led me to believe a false statement (namely, that the line on the right is longer than the line on the left).
5. If my senses made me believe a false statement once, I cannot be sure that it will not happen again.
6. My senses made me believe a false statement once.
7. Therefore, I cannot be sure that my senses won't deceive me again (i.e., make me believe false statements).

Muller-Lyer illusion

One of the ways in which empiricists have responded to this worry about the senses is by saying that information received by means of one sensory modality can be verified by information received by means of another sensory modality. For example, Locke says that we can confirm a report from one of the senses with another report from another sense (e.g., if I doubt that I see a fire, I can approach it and feel its warmth). According to Locke, "Our senses, in many cases, bear witness to the truth of each other's report concerning the existence of sensible things without us."

However, given what we now know about how the senses work, it seems that this empiricist reply will not do. Instead of confirming each other's reports, it seems that the senses often override each other's reports. For example, a phenomenon known as the McGurk effect shows that visual reports override auditory reports.

Is this a problem for empiricists? If so, how might empiricists respond to this challenge?