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?