Perception as a source of knowledge

• Direct Realism


  The immediate objects of perception are mind-independent objects and their properties.
• The argument from illusion


  P1: When subject to an illusion, an object appears to a perceiver to have a particular property.

  P2: The perceiver is directly aware of this apparent property.

  P3: The object does not have the property in reality.

  C: What the perceiver is directly aware of and what is real are distinct so direct realism must be false.
• The argument from perceptual variation


  P1: Direct realism claims that the immediate objects of perception are material objects and their properties.

  P2: But when we perceive physical objects, the appearance of their properties can vary.

  P3: The properties of the object themselves don't vary.

  C: Therefore direct realism is false as the apparent properties are not the same as the real properties of physical objects.
• The argument from hallucination


  P1: Hallucinations occur when a person perceives something which doesn't exist outside the mind.

  C1: What they perceive exists only in the mind.

  P2: Hallucinations can be subjectively indistinguishable from veridical perceptions.

  C2: Direct realism is false as what we are aware of in veridical perception must also be in the mind.
• The time lag argument


  P1: The light from distant objects such as the sun takes time to meet our eyes.

  C1: What we are seeing now may no longer exist.

  C2: What we are seeing now and what is there are different.

  P2: This is no less true for physical objects at any distance.

  C3: Direct realism is false as what we see are appearances, not physical objects.
• Indirect realism


  The immediate objects of perception are mind-dependent objects (sense data) that are caused by and represent mind-independent objects.
• Primary qualities


  Gives us ideas that resemble the qualities of the object e.g. size and shape.
• Secondary qualities


  Gives us ideas that represent the qualities of the object e.g. colour and texture.
• Support for the primary/secondary quality distinction from perceptual variation


  P1: The same water can produce the idea of cold to one hand and of warmth to the other.

  P2: The same thing cannot be both cold and warm at once.

  C: Therefore the cold and warmth cannot belong to the material object and must be purely sensations produced in the perceiver.
• Indirect realism leads to scepticism about the existence of mind-independent objects

  • Indirect realists believe we are only directly aware of sense data and can only infer the existence of objects beyond the mind.
  • Our senses can deceive us and it is possible that our sense data does not correspond with a material reality.
  • E.g. brain in a vat scenario.
  • We cannot look beyond the 'veil of perception' to confirm that a material reality exists.
• Locke's argument from the involuntary nature of our experience

  • Unlike imagination, perception is not subject to my will.
  • Therefore the source of this sensation must be external.
• The coherence of various kinds of experiences

  • Developed by Locke and Trotter Cockburn.
  • We learn to associate the way objects feel to touch with their appearance.
  • We can also predict what we will perceive next.
  • For example an apple left untouched in a drawer will go rotten.
• The external world is the best hypothesis (Russell)

  • No deductive proof of the nature of a material reality is possible.
  • However we have an instinctive belief in the existence of a material reality which corresponds with our sense data.
  • There is no significant positive evidence to suggest that belief in an external world is untenable.
  • Therefore the existence of an external world is the best hypothesis for explaining why our sense data appears in regular and predictable ways.
• Ideas cannot be like material objects (Berkeley)


  P1: My idea of a tree has certain sensible qualities e.g. shape and colour.

  P2: These sensible qualities depend on the mind.

  P3: To say that my idea of a tree resembles the real material tree is like saying something visible can resemble something invisible, or that a sound can resemble what is not a sound.

  P4: Ideas are fleeting and changing, while material objects are supposed to be permanent and unchanging.

  C: Anything outside the mind cannot have such qualities and a supposed material object could not resemble my idea of it.
• Berkeley's idealism


  The immediate objects of perception (i.e. ordinary objects such as tables and chairs) are mind-dependent objects.
• Berkeley's attack on the primary/secondary quality distinction


  P1: it is impossible to imagine an object with only primary qualities such as shape and size.

  C1: Our ideas of so-called secondary qualities cannot be separated from those of its primary qualities.

  C2: It follows that they must exist together.

  P2: Indirect realism accepts that our ideas of secondary qualities are mind-dependent.

  C3: Therefore our ideas of primary qualities must also be mind-dependent.
• Berkeley's argument from perceptual variation


  P1: What looks small to us will look big to a mite.

  P2: A material object cannot be small and big at the same time.

  C: Therefore size cannot be property of material objects.
• Berkeley's master argument 


  P1: Try to conceive of a tree which exists independently of any mind.

  P2: In doing so the tree is being conceived by you.

  C: Therefore the tree is in your mind and not independent of any mind after all.
• The argument from illusion and hallucination

  • If there is no mind-independent reality then there is nothing to separate illusions and hallucinations from veridical perception.
• idealism leads to solipsism

  • If Berkeley's reasoning is correct it seems that the world first appeared when I was born and comes in and out of existence every time I blink my eyes.
  • Although Berkeley claims God is the permanent perceiver of the universe, we cannot have an idea of a mind because a mind is the possessor of ideas.
  • This leads to solipsism which is the view that all we can be certain of is the contents of our own minds.
• Problems with the role of God in Berkeley's idealism


  P1: Berkeley claims that what we perceive is in the mind of God.

  C1: It follows that the idea of pain is in God's mind and God suffers pain.

  P2: But if God suffers pain then he is imperfect.

  P3: God is defined as a perfect being.

  C2: Therefore Berkeley's views lead to a contradiction.