Can you imagine a bachelor who is hungry or is bald? Is he still a bachelor? Obviously yes. However, if he were to get married or to have a sex change, he would no longer fulfil the criteria of bachelorhood (being a single male), so being unmarried and male are the essential elements (or primary qualities) of being a bachelor. Another example would be when we consider an object like an apple. We are still able to recognise that an apple is an apple, regardless of whether or not we can see its colour or be able to smell it or hear it. So it is obvious that these qualities are not the essential properties of an apple. However, we are unable to imagine an apple that is devoid of any shape, size, position or motion, which would lead us to conclude that these are the essential properties of an apple. We can only learn about physical objects through the observation of the primary qualities in our sense data, and as long as we are able to recognise which aspects of our sense data are accurate representations of the external world, we can build an accurate picture of it.
Representative realism does manage to avoid the argument from illusion. However, the distinction between the world as it is and the world as it is perceived creates a veil of perception between us and the external world, due to its inaccessibility to our senses. Therefore, we can only really acknowledge our representations of the world, and not what it really consists of. How do we know how accurate our representation of the real world is?
Everything we perceive must come from our five senses, which would lead us to conclude that only the secondary qualities can be perceived directly, and not the primary qualities. If the existence of physical objects (possessing primary qualities) is simply inferred from our senses, how can we be sure that our senses are accurate in their representation? Without independent access to the real world, we cannot make comparisons between our sense data and the physical objects that they represent. We are not able to establish when we are being deceived, or what the world consists of, or even if it exists it all. Thus representative realism fails to explain how we come to have knowledge of the world, or even if it is possible.
