Still less may we take appearance and illusion for one and the same. For truth and illusion are not in the object, insofar as it is intuited, but in the judgment about it insofar as it is thought. Thus it is correctly said that the senses do not err; yet not because they always judge correctly, but because they do not judge at all. Hence truth, as much as error, and thus also illusion as leading to the latter, are to be found only in judgments, i.e., only in the relation of the object to our understanding. In a cognition that thoroughly agrees with the laws of the understanding there is also no error. In a representation of sense (because it contains no judgment at all) there is no error. No force of nature can of itself depart from its own laws. Hence neither the understanding by itself (without the influence of another cause), nor the senses by themselves, can err;

– Immanuel Kant, Critique of Pure Reason

I agree that if we believe there to be a city at the horizon, which in reality is just a Fata Morgana, it’s not like the senses wrongly judge the city to be real. Reason does.

But in many cases, the senses are not just neutral yet imperfect observers of our environment. The senses do err, they do, at least metaphorically, judge.

Sure, the senses’ judgement is not like a rational judgment (when reason assents to a proposition). So perhaps I stretch the meaning of “judge” a bit here. But we can at least state: The senses (pre-)interpret our environment in a certain manner, with a bias outside of our control, and apply pressure to us to accept their interpretation.

It’s certainly the senses’ interpretation that the arrows in the Müller-Lyer illusion are of different length, not the intellect’s. Even if we rationally discover that they must be of the same length, the senses remain inveterate, attached to their error.

This seems especially true since Kant defines sensation as:

The effect of an object on the capacity for representation, insofar as we are affected by it, is sensation.

(Die Wirkung eines Gegenstandes auf die Vorstellungsfähigkeit, sofern wir von demselben afficirt werden, ist Empfindung.)

So did Kant ever explain why the “senses do not err” when it comes to optical illusions?

  • 2
    Kant has a lot of intermediaries between senses and intellect, forms of intuition, schemata, productive imagination, understanding, judgment. He is not employing colloquial meaning of "senses" or metaphors about "judging". Judgment is a conscious representation of objects, senses are just feeler pipes. The manifold of sensation is completely undifferentiated, all the "processing" is done afterwards, illusions included. Sensibility does not interpret, put any pressures or judge. What he says is a platitude.
    – Conifold
    Commented Aug 22, 2021 at 21:29
  • @Conifold how did Kant know that illusions like Müller-Lyer originate from visual input processing in the brain? This is not at all suggested by introspection (quite the contrary). So Kant’s division is based on a pure hunch. But he presents his hunch as a fact – with supreme confidence. Without neurophysiology we have no idea if the Müller-Lyer illusion originates in the organ “eye” or in the brain. If it originated in the organ “eye”, it is an illusion caused by the senses.
    – viuser
    Commented Aug 23, 2021 at 20:57
  • He didn't need to know that. He is concerned with faculties that take part in synthesizing perception, not with organs that implement them, and he is backengineering what has to be done functionally to enable the sort of perception we exhibit, not introspecting visual cortex. Whether this or that function is performed in the brain, in the eye, or partly in each makes no difference to his concerns. The faculty of sensibility does no judging or imagining whether the organ eye participates in them or not. He can be confident because he is not doing physiology or psychology of perception.
    – Conifold
    Commented Aug 23, 2021 at 23:07
  • Kant uses most of his terms in a specialized manner, as any reader of his quickly discovers. And analysis of a complex function is useful even in abstraction from how the function is implemented. We do not fault theory of algorithms for not identifying the types of CPU and memory used, or data exchange protocols between them. Kant's quip is not really a claim, it is a clarification of terminology. And modern psychologists found parts of his analysis of cognitive functions insightful and helpful, see SEP and Brook's book. That's one reason why.
    – Conifold
    Commented Aug 23, 2021 at 23:50

1 Answer 1


When a person looks at a picture displaying the Müller-Lyer illusion, there is a 6 steps process that happens:

  1. Neurons in the eye detect light.
  2. The eye gives this information to the subconscious.
  3. The subconscious processes this information.
  4. Conclusions of the subconscious are sent to the conscious.
  5. The conscious decides based on these conclusions that the arrows are of different length.
  6. The person says "The arrows are of different length."

Kant grouped steps 1 and 2 together as 'senses' and steps 3 to 5 together as 'mental processes.' What Kant is saying is that, in step 6, the person's vocalization of an incorrect claim about the arrows happened because, somewhere in the mental processes, an error was made.

In contrast, you grouped steps 1 to 3 as 'senses' and steps 4 to 5 as 'mental processes.' Because step 3 is actually where the error was made, this change in definitions resulted in you thinking Kant was "obviously wrong."

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