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13. Knowledge Criteria / D. Scepticism / 3. Illusion Scepticism

[our confidence in perception is undermined by illusions]

18 ideas
Illusions are not false perceptions, as we accurately perceive the pattern of atoms [Epicurus, by Modrak]
     Full Idea: Epicurus says illusions are not false perceptions, because the senses accurately report the pattern of atoms; for instance, the edges are worn off the pattern produced by a square tower, so its perception as a round tower is true.
     From: report of Epicurus (Letter to Herodotus [c.293 BCE], 47-53) by Deborah K.W. Modrak - Classical theories of Mind
     A reaction: As so often, Epicurus got it right, because Democritus got it right, thus demonstrating that good philosophy must be preceded by good physics. However, good physics must be preceded and followed by good philosophy.
Epicurus says if one of a man's senses ever lies, none of his senses should ever be believed [Epicurus, by Cicero]
     Full Idea: Epicurus says that if one sense has told a lie once in a man's life, no sense must ever be believed.
     From: report of Epicurus (fragments/reports [c.289 BCE]) by M. Tullius Cicero - Academica II.25.79
A false object might give the same presentation as a true one [Arcesilaus, by Cicero]
     Full Idea: Arcesilaus said that no presentation proceeding from a true object is such that a presentation proceeding from a false one might not also be of the same form. This is the one argument that has held the field down to the present day.
     From: report of Arcesilaus (fragments/reports [c.275 BCE]) by M. Tullius Cicero - Academica II.24.77
Most supposed delusions of the senses are really misinterpretations by the mind [Lucretius]
     Full Idea: Paradoxical experiences (such a dreams and illusions) cannot shake our faith in the senses. Most of the illusion is due to the mental assumptions we ourselves superimpose, so that things not perceived by the senses pass for perceptions.
     From: Lucretius (On the Nature of the Universe [c.60 BCE], IV.462)
     A reaction: Some misinterpretations of the senses, such as thinking a square tower round, are the result of foolish lack of judgement, but actual delusions within the senses, such as a ringing in the ears, or a pain in a amputated leg, seem like real sense failures.
Every true presentation can have a false one of the same quality [Cicero]
     Full Idea: [The sceptical Academics say] what is false cannot be perceived, but every true presentation is such that there can be a false presentation of the same quality.
     From: M. Tullius Cicero (Academica [c.45 BCE], II.40)
     A reaction: It was the stoics who focused the discussion on 'presentations'. This claim is purely theoretical; no one has ever experienced a false presentation of talking to a family member that was as vivid as the real thing.
The same oar seems bent in water and straight when out of it [Sext.Empiricus]
     Full Idea: The same oar seems bent when in the water but straight when out of the water.
     From: Sextus Empiricus (Outlines of Pyrrhonism [c.180], I.119)
The necks of doves appear different in colour depending on the angle of viewing [Sext.Empiricus]
     Full Idea: The necks of doves appear different in hue according to the differences in the angle of inclination.
     From: Sextus Empiricus (Outlines of Pyrrhonism [c.180], I.120)
The same tower appears round from a distance, but square close at hand [Sext.Empiricus]
     Full Idea: The same tower appears round from a distance, but square close at hand.
     From: Sextus Empiricus (Outlines of Pyrrhonism [c.180], I.32)
If we press the side of an eyeball, objects appear a different shape [Sext.Empiricus]
     Full Idea: When we press the eyeball at one side the forms, figures and sizes of the objects appear oblong and narrow.
     From: Sextus Empiricus (Outlines of Pyrrhonism [c.180], I.47)
Only judgement decides which of our senses are reliable [Descartes]
     Full Idea: Sense alone does not suffice to correct visual error: we also need a degree of reason to tell us that we should believe the judgement based on touch rather than vision. Since we don't have this power in infancy, it must be attributed to the intellect.
     From: René Descartes (Reply to Sixth Objections [1641], 439)
We correct sense errors with other senses, not intellect [Mersenne on Descartes]
     Full Idea: Owing to refraction a stick which is in fact straight appears bent in water. What corrects the error? The intellect? Not at all; it is the sense of touch.
     From: comment on René Descartes (Meditations [1641], §1.18) by Marin Mersenne - Objections to 'Meditations' (Sixth) 418
It is prudent never to trust your senses if they have deceived you even once [Descartes]
     Full Idea: The senses are sometimes deceptive, and it is a mark of prudence never to place our complete trust in those who have deceived us even once.
     From: René Descartes (Meditations [1641], §1.18)
The senses can only report, so perception errors are in the judgment [Gassendi on Descartes]
     Full Idea: Although there is deception or falsity, it is not to be found in the senses; for the sense are quite passive and report only appearances, which must appear the way they do owing to their causes. The error or falsity is in the judgement or the mind.
     From: comment on René Descartes (Meditations [1641], §1.18) by Pierre Gassendi - Objections to 'Meditations' (Fifth) 332
If pain is felt in a lost limb, I cannot be certain that a felt pain exists in my real limbs [Descartes]
     Full Idea: I have heard it said by people whose arm or leg has been amputated that they still sensed pain in the lost limb. Thus it does not seem certain that one of my bodily members causes me pain, even though I sense pain in it.
     From: René Descartes (Meditations [1641], §6.77)
It is possible that we could perceive everything as we do now, but nothing actually existed. [Berkeley]
     Full Idea: We might perceive all things just as we do now, though there was no matter in the world.
     From: George Berkeley (Three Dialogues of Hylas and Philonous [1713], II p.209)
     A reaction: An old Greek argument. Now we have an explanation of experience, but we wouldn't if nothing existed. Which doesn't prove that anything exists. Is some explanation always preferable to none? Cf. religion.
Examples of illusion only show that sense experience needs correction by reason [Hume]
     Full Idea: Trite sceptical examples, such as the oar bent in water, or double images when the eye is pressed, are only sufficient to prove that senses alone are not dependable, but we must correct their evidence with reason.
     From: David Hume (Enquiry Conc Human Understanding [1748], XII.I.117)
Psychologists say illusions only occur in unnatural and passive situations [Lowe]
     Full Idea: Psychologists point out that illusions almost always occur in unnatural environments in which subjects are prevented from exploiting the natural interplay between perception and action.
     From: E.J. Lowe (Introduction to the Philosophy of Mind [2000], Ch. 6)
     A reaction: It has always struck me that philosophers make a great deal out of illusions, but I don't think I have ever had one. I don't know anyone who has seen a non-existent dagger.
Illusions are not a reason for skepticism, but a source of interesting scientific information [Sorensen]
     Full Idea: Philosophers tend to associate illusions with skepticism. But since illusions are signs of modular construction, they are actually reason for scientific hope. Illusions have been very useful in helping us to understand vision.
     From: Roy Sorensen (Vagueness and Contradiction [2001], 1.4)
     A reaction: This is a nice reversal of the usual view. If I see double, it reveals to me that my eyes are not aligned properly. Anyone led to scepticism by illusions should pay more attention to themselves, and less to the reality they hope to know directly.