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The argument from illusion against direct realism is almost always phrased in terms of something like a stick in a puddle appearing bent. I find this very unconvincing given that the misleading part of it is in the world, that is, refraction. Something like the Müller-Lyer illusion on the other hand seems to have all the ambiguity in the perceiver's mind.enter image description here

The two horizontal lines are the exact same length, and it's only the arrows on the end that make you think otherwise, and yet, unlike refraction, these arrows don't actually change how the light moves or anything like that which would change the appearance of the horizontal lines to some kind of unbiased observer. So, how does the direct realist explain this? It seems impossible for them to explain it given that it's a bias purely in the brain affecting our perception.

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    Why would you thunk that Müller-Lyer illusion is also not "in the world", since the bottom arrows point outward thus giving the impression of and actually being larger? Anyway, the direct realist can possibly claim that these illusions can be eventually found out, through (rest of) reality
    – Nikos M.
    Commented May 30 at 11:07
  • @NikosM. Well because that appearance is only due to the fact that we have learned to associate certain things with depth cues. In the 70s, some anthropologists found that a zulu tribe saw the horizontal lines as the same length. The mis-perceiving seems to be purely due to our experience rather than out in the world. I don't think it's cultural conditioning which makes us see refraction
    – edelex
    Commented May 30 at 11:28
  • The direct realist can possibly claim that the length is fixed and this can be known directly (eg by measuring it). Furthermore the realist may claim that at first glance indeed one shape is larger and this is also known directly and true. The choice of focusing on which part being free or fixed by other parameters is not any problem for the direct realist.
    – Nikos M.
    Commented May 30 at 11:37
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    It is easier for direct realists to explain it exactly because the bias is "in the brain" and not "in the eye". We perceive both images perfectly faithfully, it is that we judge their comparative length mistakenly. And when this is pointed out and we re-examine our perceptions, we can ascertain that the segments are, in fact, of the same length. The mistake is no fault of perception. Direct realists only claim that what we perceive is real, not that we judge and interpret it without mistakes.
    – Conifold
    Commented May 30 at 11:37

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Following Maloney (2018) we can say that evaluating the length of the lines relative to one another (or the continuity of the stick in water) is introducing a different level of cognition, beyond mere perception itself. The mind accurately percieves sense data from the physical world, but can potentially make errors in processing or analyzing that perception. This narrower definition of what perception is may diverge a bit from how most psychologists normally use the term, but that does not automatically invalidate the distinction.

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John Searle, a famous direct realist, in his Seeing Things as They Are (GB) addresses the question of various versions of the Argument from Illusion. A lightweight answer to your question comes form Seale arguing what Austin argued. Starting on page 89, The bent stick and the elliptical coin, he explicates:

The argument against Direct Realism in these cases claims that I did actually see *something bent and something elliptical... I saw the bent appearance of the stick and the elliptical appearance of the coin, so the elliptical and bent appearances were the objects of my perception... I do not see objects but only sense data.

Of course, the idea that there is an intermediate representation Searle objects to. He calls it the Bad Argument throughout the book, and offers a different theory of perception to eliminate it as the thesis of his book. In the case of the illusions of the bent stick and the elliptical coin, he goes on to say:

The literal false step in the argument is the one that says: because I directly perceive something elliptical and because the coin itself is not elliptical, it follows I do not directly perceive the coin. But it does not follow... There is no way I can see the appearance of the coin without seeing the coin... it does not follow that I do not see the coin.

What Searle doing here is challenging the notion that the coin from one perspective is not the coin (the elliptical coin) while it is from another direction (the round coin). We'll say more about this momentarily:

But this is not to describe an actual bent or elliptical object of his perception but rather the conditions of satisfaction of a perceptual experience... which has intentional content which may or may not be satisified, and the actual object and state of affairs... perceived with varying degrees of accuracy.

So, my interpretation of this would include the terms intension and extension borrowing from Frege's intellectual heirs. Obviously, we have the same referrent, the same extension involved in the analysis, the coin. But as I see it, what Searle does is challenge the idea that one intension of the coin, the one involving round property, is somehow 'real' and the other intension, the one involving the elliptical property, is somehow 'appearance'. Both of them are directly the coin, but perceived, as he says, under different "conditions of satisfaction of a perceptual experience".

Think about it. In the question of Hesperus and Phosporous, which is actually Venus? Should we accept that Morning Star is the real Venus, and the Evening Star is a mere appearance? No, we simply accept that in both cases, we observe Venus, but under different times and conditions. In both cases, we observe Venus directly.

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  • "In both cases, we observe Venus [or the coin] directly." What does the word "directly" mean here? What work does it do? We certainly observe a coin or a planet, but what does it mean to observe it "directly"?
    – Olivier5
    Commented May 30 at 16:34
  • @Olivier5 Searle's direct realism objects to representational theories of mind. "The representational theory of mind attempts to explain the nature of ideas, concepts and other mental content in contemporary philosophy of mind, cognitive science and experimental psychology. In contrast to theories of naïve or direct realism, the representational theory of mind postulates the actual existence of mental representations which act as intermediaries between the observing subject and the objects, processes or other entities observed in the external world...
    – J D
    Commented May 30 at 18:58
  • These intermediaries stand for or represent to the mind the objects of that world. " - en.wikipedia.org/wiki/… Directly is the adverb by which we discount indirect realism. Consider Kant's Das Ding an sich. en.wikipedia.org/wiki/Thing-in-itself
    – J D
    Commented May 30 at 18:58
  • Considering Searle's insistence, in the book you linked to, on states of affairs functioning causaly in producing perceptual experience, it seems that for Searle, a causal link must exist between the objective thing being observed and its observation (subjective perception by a human being). If memory serves, for vision this causal link includes photons, a dedicated, complex organ called an eye, which functions like a camera, chemical photo-receptors triggered by certain precise wavelengths, and millions of neurons processing the image in real time. Isn't all this "intermediate"?
    – Olivier5
    Commented May 30 at 20:31
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    @Olivier5: This is more or less the same objection that people raise against Searle's Chinese Room argument. IMHO the more successful objection is (perhaps surprisingly) to directly accuse Searle of property dualism, which apparently upset Searle enough that he felt the need to write an entire essay rebutting it. Personally, I did not find this essay overly persuasive, but I would encourage reading it as it goes into this subject matter in some detail.
    – Kevin
    Commented Jun 2 at 4:47

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