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‘We should note here that the intellect can never be deceived by any experience, provided that when the object is presented to it, it intuits it in a fashion exactly corresponding to the way in which it possesses the object, either within itself or in the imagination. Furthermore, it must not judge that the imagination faithfully represents the objects of the senses, or that the senses take on the true shapes of things, or in short that external things always are just as they appear to be. In all such cases we are liable to go wrong, as we do for example when we take as gospel truth a story which someone has told us; or as someone who has jaundice does when, owing to the yellow tinge of his eyes, he thinks everything is coloured yellow; or again, as we do when our imagination is impaired (as it is in depression) and we think that its disordered images represent real things. But the understanding of the wise man will not be deceived in such cases: while he will judge that whatever comes to him from his imagination really is depicted in it, he will never assert that it passes, complete and unaltered, from the external world to his senses, and from his senses to the corporeal imagination’ (Rules for the Direction of the Mind, Rule 12) \citep[p.~47, AT X:423]{descartes:1985_csm1}.

‘consider the reasons why [vision] sometimes deceives us. First, it is the soul which sees, and not the eye; and it does not see directly, but only by means of the brain. That is why madmen and those who are asleep often see, or think they see, various objects which are nevertheless not before their eyes: namely, certain vapours disturb their brain and arrange those of its parts normally engaged in vision exactly as they would be if these objects were present. Then, because the impressions which come from outside pass to the 'common' sense by way of the nerves, if the position of these nerves is changed by any unusual cause, this may make us see objects in places other than where they are ... Again, because we normally judge that the impressions which stimulate our sight come from places towards which we have to look in order to sense them, we may easily be deceived when they happen to come from elsewhere. Thus, those whose eyes are affected by jaundice, or who are looking through yellow glass or shut up in a room where no light enters except through such glass, attribute this colour to all the bodies they look at. And the person inside the dark room which I described earlier attributes to the white body the colours of the objects outside because he directs his sight solely upon that body. And if our eyes see objects through lenses and in mirrors, they judge them to be at points where they are not and to be smaller or larger than they are, or inverted as well as smaller (namely, when they are somewhat distant from the eyes). This occurs because the lenses and mirrors deflect the rays’ (Optics) \citep[pp.~172--3, AT VI:141--2]{descartes:1985_csm1}.