However, one reply to this objection is that the existence of the external world is a plausible hypothesis. Representative realists can counter this objection by saying that, although we can’t know for certain that the external that the external world exists, its existence is a plausible hypothesis. An example is given by philosopher Bertrand Russell to support this point: If a cat appears at one moment in one part of the room, and at another in another part, it is natural to suppose that it has moved from the one to the other, passing over a series of intermediate positions. But if it is merely a set of sense data, it cannot have ever been in any place where I did not see it; thus we shall have to suppose that it did not exist at all while I was not looking, but suddenly sprang into being in a new place. If the cat exists whether I see it or not, we can understand from our own experience how it gets hungry between one meal and the next; but if it does not exist when I am not seeing it, it seems odd that appetite should grow during non-existence as fast as during existence.
Essentially the point that Russell is raising here is that the simplest way of explaining the coherence and regularity of our sense-data is to simply say that they are caused by physical objects that exist independently of us. If Russell’s theory is right then we can infer the existence of the external world from our sense-data, in exactly the same way as scientists infer the existence of unobservable entities like electrons from the experimental data that they observe.
Another criticism to representative realism is that; if what we perceive of the world is a form of representation of the world, then how can we be sure how accurate that representation is? To put it simply, how can we be sure that the human perception of the actual world that is being perceived resembles it in any way or form? Therefore the only things we ever perceive are secondary qualities; we never perceive primary qualities directly. Absolutely everything we perceive must come from our five senses. Hence we have to deduce the existence of an independent object with primary qualities solely based on our awareness of our sense impressions. However, if the existence of physical objects if deduced from our sensations how can we are certain they are an accurate representation? Without actual independent access or a “god’s eye view”, from which to observe both our sense impressions and the real world, it is impossible to know if we are being deceived or how, accurate our representations are.
However, one response to this criticism is that sense data must just be as they appear. The sense-data don’t genuinely have shape, size, colour etc., but just appear to. However, saying this would conflict with one of the key differences between sense-data and physical objects, which is that sense data are always just as they appear. This difference between sense data and physical objects creates further problems for representative realists. For if sense-data are just as they appear, then people who perceive them must be aware of all of their properties. Yet, we don’t seem to be aware of all of the properties of our sense-data. If you think of the sense-data you see when looking at a pile of matches on a table, you are not aware of how many ‘match images’ there are in this mental image. If we were, then we would easily know straight away how many matches there were in the pile, however this doesn’t happen.
Furthermore, another criticism made against representative realism is that we cannot even talk about the real world. Our mind receives data from this “real” world, which it then processes. This post-processed world is the only thing we have some form of entry into. All the concepts and words we learn during our lifetime are learnt by dealing with the world as it is perceived. So, our concepts are designed to match and work in this world of sensation. However there is no reason whatsoever why out concepts should apply in the world as it really is. Some might say that the real world is the cause of our sensation. But, can this be said because causation is a concept that (as far as we know), applied only to world of sense experience. We cannot observe the real world causing the perceived world, nor can we assume that the ‘real world’ shows any casual relationships whatsoever, or whether it is spatial-temporal. We can’t imagine such a world which has no space or time or causality, but this is to be expected since the real world lies totally beyond our comprehension.
In conclusion, I think that the even though many objections are made against indirect realism, there are fairly strong responses to them as well. The problem of representative realism leading to skepticism suggests that since we have no way of checking whether the sense data we perceive are accurately representing the ‘external world then we can’t know what the real world is like. A strong response is given to this point and it states that even though we can’t know for certain that the external that the external world exists, its existence is a plausible hypothesis, and this response is supported by Russell’s cat analogy. Also, there are other strong responses to other objections such as the point made saying that sense data are impossible. The response here states that sense data are just as they appear. The sense-data don’t genuinely have shape, size, colour etc., but just appear to.