I have been challenged to articulate the argument for indirect realism, so thought I would post some further thoughts and some conclusions now I have completed the first perception module.
Indirect realism rests on the key fact that our perceptions are not the same as the object perceived. I would admit this as true in one sense, there is a conceptual difference between my perception of the stick and the stick itself.
Indirect realists believe that what we perceive is not the object, but sense data (or as Berkeley puts it "ideas"). Locke would argue that secondary qualities are ideas caused in us by the power of the object to produce those ideas. Indirect realists therefore argue that all we are aware of are mental representations. These mental representations act as intermediaries between the subject and the object being perceived. Since we cannot directly perceive the object, we have to infer its existence from the mental representations.
I am struggling to find a coherent and intelligible explanation of what a sense-datum is. Stanford encyclopedia says they are "mind-dependent objects that we are directly aware of in perception" but this doesn't offer any real kind of definition. I am afraid they remain very obscure from my perspective.
Having read some more arguments in favour and against indirect realism, I think I would have to classify myself as a naive realist. I feel that I have made a Moorean shift, and would want to argue from the belief that we do directly perceive the external world to a philosophical explanation of what we mean by this. To me it seems both self-evident and coherent that we directly perceive the external world, but I'm not sure which of the two justifications comes first.
We perceive things differently in different circumstances, but this seems to me to be consistent with a direct realist view. The stick appears bent under certain circumstances, i.e. we directly perceive the stick as it appears.
Talk of the stick appearing "as it really is" is, I believe, meaningless. The stick can only appear to us as it appears. The stick really is how we perceive the stick, assuming we are not suffering from a perceptual hallucination. It is a stick which appears bent under water and tiny when seen a long way away and red when seen in red light. We don't therefore perceive sense-data, we perceive the stick as it appears.