Why is it that as one is not presented with the ordinary object in an illusion, the same account holds for veridical experience?

Question

The Stanford Encyclopedia of Philosophy contains the following interpretation of the Argument of Illusion in the context of the problem of perception:

In an illusion, it seems to S that something has a sensible quality, F, which the ordinary object supposedly being perceived does not have. When it seems to S that something has a sensible quality, F, then there is something directly presented to S which does have this quality. Since the ordinary object in question is, by hypothesis, not-F, then it follows that in an illusion, S is not directly presented with the ordinary object supposedly being perceived. The same account of experience must apply to both veridical and illusory experiences. Therefore, in veridical experience, S is not directly presented with the ordinary object supposedly being perceived. If S is not directly presented with the ordinary object supposedly being perceived in veridical experience, S is never directly presented with an ordinary object.

I do not quite understand the link between us not being directly presented with the ordinary object in illusions and its case in all of our veridical experiences. The source further mentions the Common Kind Claim, in which "for any veridical perception of an ordinary object, we can imagine a corresponding illusion or hallucination which cannot be told apart or distinguished, by introspection, from the veridical perception".

How can the common kind claim then be used to establish the conclusion that we are never directly presented with ordinary objects, since the common kind claim merely states that we can hypothetically come up with an illusionary experience in which we are presented with the same set of sense datum as in the veridical experience? For instance, when I perceive a red flower, what kind of corresponding illusion can I imagine which cannot be told apart from veridical perception? I understand that when I look at a straw submerged in water, my sense datum is of a bent straw which is an illusionary experience. However, when I perceive a red flower, am I not directly presented with an ordinary object of a flower that is red?

Answer 1

The idea here is that we don't have any effective way to bridge between object and perception. In other words, if we see a red rubber ball the phenomenon we perceive is the impact of light on the retinas of our eyes, which is then organized and structured by physical and mental processes to give us the impression of a red rubber ball. The object itself is merely presumed to exist somewhere 'out there' beyond the bounds of our senses. The same can be said for any perception: touch, taste, sound, smell. There is always some mediating factor between the external object and the internal representation, and any mediating factor can be interrupted or hijacked.

• If we put a red rubber ball next to a hologram of a red rubber ball, can we tell which is which by sight?
• If we hear a red rubber ball bounce, can we tell whether it was a real ball or a synthesized sound by hearing?
• If we hallucinate a red rubber ball, can we know that that it is a hallucination?

The problem is that our senses are our only way of verifying and validating the world, so if our senses are deceived we cannot verify or validate the deception. It's the dark sordid secret of empiricism...