Indirect realism is an easy position to argue for. Simply consider the actual mechanics of perception (say visual perception). First there is the fact that objects never directly present themselves to our eyes, only photons reach us, and many things could have generated those photons, not just the object we think that we see. Even the information generated by photons striking the eye is not directly incorporated into consciousness. Our unconscious preprocesses this information, which we can deduce from the existence of optical illusions and the fact that concepts are present as part of visual experience (when you see a tree you don’t just have a certain visual sensation, you see a tree). Thus it is only information about the external world, transformed several times, that becomes part of consciousness, and what better word for it could there be than sense data?
But wait! Didn’t I give a defense of direct realism earlier? Which one is right? Well they both are. Yes both direct and indirect realism are correct, what seems like real differences between them are simply different ways of talking about the same phenomena. When you ask an indirect realist what experience is about he or she is likely to say that it really about the sense data that is part of consciousness. On the other hand when you ask a direct realist what experience is about they will say it is really about objects in the external world that cause our experiences. What we have here is not a real disagreement, but a disagreement as to what about-ness in perceptual experience is. In theory neutral terms an indirect realist uses “about” to designate the part of consciousness that is responsible for the perceptual experience, and the direct realist uses “about” to designate the causes of the experience.
So, given that direct realism and indirect realism are compatible with each other, and that their apparent incompatibility is a result of using the same word in different ways, where does that leave us? One concern that must be addressed is whether indirect realism naturally leads to idealism (or solipsism), because if it does, and if direct realism is not an alternative position but a complementary position, then it seems we might be forced to be idealists. The argument goes as follows: if our experiences of two people, or the experiences of a single person at different times, aren’t really about the same thing, (in indirect realist terms) then what is to prevent us from concluding that the entire world we know is simply a construction by our minds, and that there is nothing real behind it all?
What we need then is an argument against idealism that doesn’t rely on direct realism. That argument is as follows: We notice a constancy in our sensory experience; when we look at things from different angles, when we examine them with different senses, and when we observe them at different times, in all of these cases our senses give basically consistent information about the object that we are supposedly perceiving. The most likely explanation is that there is something causing this consistency, something that is not part of our conscious minds. We simply call that something “physical reality” (although it really could be anything, even a computer simulation). Additionally our experiences of “physical reality” seem to correlate with the reported experiences of other people, and once again this implies it is likely that “physical reality” is the same for all people. From this we can conclude that the most likely explanation (by far) for our experience is that there is something external to us that is responsible for the consistency of experience, something that we call reality, for lack of a better word.
Thus, with the threat of collapsing into idealism put aside for the moment, direct and indirect realism can coexist without problems. Of course the disagreement as to how exactly we should use the word “about” in philosophy hasn’t been resolved, but “about-ness” is complicated in its own way, more than the direct/indirect realism debate reveals, and thus its exact nature will have to be left for another day.