This is the the seventh post in my Theories of Knowledge and Reality series. Follow the link for more on the series and for links to other entries as they appear.
How perception works is relevant to skepticism about knowledge through the senses, so it might be nice to get a little background on theories of perception. There are three general views in the history of philosophy on the nature of perception. I'm going to be talking in terms of three prominent philosophers in the earlier modern period who held the three diffferent views. How you interpret some of the responses to skepticism will depend in some cases on which view of perception you have (and one of them, as we saw in the last post, is itself a sort of response to skepticism).
[Note: I'm less confident with this post than with some others that I'm representing the historical figures as carefully as I'd like to represent them. In some ways these figures are standing for the overall view, and I'll sometimes refer to a contemporary response as if it's what the historical figure would say. I'm not really pretending to be accurate to the historical figure when I do this. I'm more trying to explore the view. I do think most of what I say is close enough to what they say, but I don't want to look as if I'm doing history of philosophy. This post is just to get a sense of what these views can look like.]
One way to understand the three views is to consider the following inconsistent triad of claims:
1. We perceive ordinary objects.
2. What we perceive are ideas (something internal to our minds).
3. Ordinary objects exist outside of us -- external to our minds.
Any view will have to deny at least one of these claims. If 1 and 2 are true, 3 comes out false. If 1 and 3 are correct, 2 must be false. If 2 and 3 are right, then 1 must be wrong. This is how the three views we are considering will work. These three claims are inconsistent because they can't all be true.
Rene Descartes and George Berkeley both affirm 2. The things we perceive are ideas. Descartes will say that these ideas represent things in the external world, and Berkeley will deny that there are any external things to be represented. To affirm 3, Descartes has to give up 1. Berkeley wants to affirm 1, though, so he ends up denying 3.
Both views have problems. Descartes' view that we perceive ideas of things easily leads to skepticism. Ideas are ideas of real things, but we just perceive the ideas, not the things they represent. (This view is a sort of realism, since Descartes says that external objects are real, but it is called indirect realism, since we perceive only what represents the external things. The skeptical problem is that we have no way to know if our ideas are caused by an external world. If all we perceive are ideas, but ordinary objects exist beyond our perception, then how could we know the external world exists?
This problem leads nicely into Berkeley's idealism. He denies that external objects exist. There's no reason to postulate these extra material objects. All we perceive are ideas, and if we don't need anything else for our common sense statements to be true, then he isn't going to worry about anything external to the mind. He then gives a theory of how our words are just about ideas, because the things we perceive are ordinary objects - ordinary objects are just ideas. So when we talk about ordinary objects, we're talking about ideas. We're talking about chairs, yes, but a chair is just a collection of perceptual ideas in my mind. So Berkeley has given a whole theory that doesn't need any external material objects.
Thomas Reid came along a bit later and wanted to give an account of perception that didn't get the consequence that there aren't any external objects and didn’t get the skeptical worry either. His brilliant solution was to notice that both Descartes and Berkeley were assuming 2. They were assuming that what we see are ideas. If we deny 2 and say that we don't perceive ideas but just directly perceive material objects, then we avoid saying the wacky things Berkeley has to say.
We also don’t have to worry about the questions that concerned Descartes. We don’t have to explain how we could know that our ideas directly represent the external material objects. We just see the external objects. A causal process takes place. Light bounces off objects, and it causes us to see the objects. Sometimes this goes wrong, and we see the objects in unusual ways, like the straight stick looking bent in water or the round tower looking square from a distance. Yet we're still just seeing the object - just wrongly. This is often called direct realism, since we see directly whatever objects there are in the external world.
Many contemporary philosophers, and pretty much every major philosopher in Reid's own time, would say that he just couldn't be right. He was trying to have his cake and eat it too. The reason is that it just seemed obvious to his contemporaries that there's something that gets between us and the world. There’s this causal process that takes place as light, sound waves, particles in our nose, etc. are getting into our system. These other things are the things that get us into sensory awareness of things in the world. So these objectors to Reid wanted to say that these causal processes come between us and the world. Given that, they wanted to insist that Reid's view must be wrong. We just perceive the ideas we get as these particles, sound waves, light, etc. enter our system.
Reid's response was not that we perceive things without any causal process going on between us and what we're perceiving. His view is that the causal process makes us perceive the thing itself. He might even say that, when we see President Bush on TV, we’re seeing him. There's certainly a TV screen that we're perceiving, but we are seeing him through seeing his image on the TV. Similarly, we are in certain perceptual states, and these states are caused by things in the world, so we are seeing, hearing, feeling things in the world -- through having perceptual states. We don't perceive perceptual states. We're in a state of having a perceptual state. It's just like any other mental state. I can be in a state of pain, but it sounds funny to say that I'm perceiving a pain state in me. Reid would say that it's just as funny to say that I'm perceiving my idea (or image) of the jar of peanuts in front of me. No -- I'm perceiving the jar of peanuts, and my perceptual state is just some state I'm in. I don't perceive that state. I'm just in the state.
There are still problems Reid has to account for. If we don't perceive images, then what do we perceive when we hallucinate or dream? A direct realist might say that in strange exceptions, such as hallucinations or dreams, we perceive images. Or she might say that we still perceive whatever object is there - even if it is empty air. We just misperceive it. Either way, a direct realist has at least something to say.