How do empiricists like Hume explain the existence of perceptual illusions? If we only know about the world through sense impressions, how can we know that an object is not as it seems? For example, look at the Müller-Lyer illusion shown below. The bottom line appears longer than the line on top. But in reality they are exactly the same length when you measure them with a ruler. But why trust the perception of the ruler measuring both to be the same length any more than the two lines appearing to be of different lengths?

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  • 4
    Because out perception of external reality is "processed" by the brain. Dec 19, 2023 at 12:42
  • in empirics(/reality) there is(/exists) no "same length" (neither "lines", nor "parallels", nor "points", nor "circles" ...all approximations)
    – xerx593
    Dec 19, 2023 at 12:45
  • 3
    How would it be discovered that that's an illusion if not for empirical evidence?
    – TKoL
    Dec 19, 2023 at 14:39
  • 1
    You would have to consider the ruler stretches and shrinks depending on what line it's close to, but only when those lines have those arrow shaped forms at their ends. Which in itself is... kind of a stretch?
    – armand
    Dec 20, 2023 at 3:40
  • I'm not sure about Hume but illusions are an important empirical tool for studying how the human brain (which includes parts of the eyes) works.
    – JimmyJames
    Dec 20, 2023 at 16:23

5 Answers 5


I think that empiricists would not deny that they may be deceived. Indeed, philosophy of science generally holds an "all models are wrong" sort of attitude.

The key defining aspect of empiricism isn't the ability to explain a possible illusion. It is the contention that non-empirical methods would almost certainly be worse at predicting whether something were an illusion, and would definitely be worse over a long enough timeline. In other words, empiricists do not believe that empiricism provides perfect information about the world: they merely believe that among systems for acquiring information about the world, "observing it" is the best available system.

  1. Cut-out the two figures and place them side-by-side. You can verify that the two lines have the same lenght.

    This fact speaks against the original visual appearance of different lenght.

  2. We prefer the result obtained with the ruler because we used the ruler in many situations successfully and without any problem. We distrust the visual appearance because we know about optical illusion. In the present case we even know that the Mueller-Lyen illusion is due to the two additional arrows.

  3. But also the ruler is not the final word. We know from Special Relativity that the measured length depends on the relative speed between the object to be measured and the measuring observer.


How do empiricists explain the existence of perceptual illusions?

We realise that an illusion is an illusion through subsequent experience falsifying the illusion, if any. An empiricist may well never realise that an illusion is an illusion.

why trust the perception of the ruler measuring both to be the same length any more than the two lines appearing to be of different lengths?

Presumably, common sense. In the Müller-Lyer illusion, we cannot move the two lines close together, but we can move a ruler close to each line in turn.

Ultimately, we have to make a choice between believing the illusion or believing some other perception which says that the illusion is an illusion.


Let's take a moment and point out there is a big difference between the classical empiricism of Hume and the British and modern empirical thinking found in naturalized epistemology (SEP) or in scientific theory. The former is a set of philosophical propositions that are relatively primitive compared to our modern empirical methods. In science today, visual illusions are explained by perceptual theory which is a sub-discipline of psychology and cognitive science. Hence, visual systems are largely understood and modeled in the language of computation and biology. Hume, however, had none of that at his finger tips, and I know of no explanation of illusion. But if there is, it would be from his understanding of imagination (IEP). From the article:

One of the main discoveries that Hume claims to make, as a scientist of man, is that “men are mightily govern’d by the imagination.” He argues that the faculty of imagination is responsible for important features both of each individual human being’s mind and of the social arrangements that human beings form collectively.

Thus, perhaps he understood an illusion as an idea that ultimately is replicated from the senses where either the idea or the sense has in some general way made a mistake. He, like Locke was big in moving towards a representational theory of mind using sense data (SEP) and knew that the senses were fallible, that mental impressions were constructed from them fallibly, and that like causality, some ideas were heavily constructed by the mind. Ultimately, empirical psychology began with Wilhelm Wundt in Germany many decades later who started studying perception in a rigorous experimental way for the first time in history. Hume died in 1776, and Wundt made it into the early 20th century, so Hume likely wouldn't have had anything that resembles a scientific explanation of illusions.


On one hand, you're speaking about perceptual illusions, and on the other hand you're speaking of a ruler -- something that has been touched, measured. The illustration you shared is a an example of a visual illusion. Visual illusions are sort-of miscalculations made by our brains which, visually speaking, are adapted to a sort-of predictive pre-processing of the world around us. Sometimes our brain gets it wrong. This is different than touching, holding, measuring a ruler used to measure the horizontal lines shown in your illustration. So, you're kind of comparing apples and oranges. Anil Seth outlines the visual illustration you've pointed out, really well here: https://www.youtube.com/watch?v=lyu7v7nWzfo

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