Slideshow: Perception as a source of knowledge

Perception as a source of knowledge

• This slideshow will cover the following:
• 1. Direct realism
• Objections to direct realism (and their responses)
• 2. Indirect realism
• Objections to indirect realism (and their responses)
• 3. Berkeley's idealism
• Objections to Berkeley's idealism (and their responses)

Direct realism

• Direct realism is a theory of perception.
• Like all realist theories of perception, it claims that the external world exists.
• Direct realism argues that we perceive all physical objects as they are (we perceive physical objects directly).
• This means that all of our perceptions, according to direct realism, are 100% true and infallible.
• Also, this infers that we have a direct sense experience of the external world, which is made out of sense data.
• This sense data makes up our mental image of the external world (our perceptions); they tell us of the world around us. For direct realism, our sense data is 100% true.
• As physical objects are exactly as we perceive them (in accordance with what direct realism claims), there is no difference between appearance and reality.
• Therefore, with direct realism, there is no appearance-reality distinction.

Objection 1: Perceptual variation

• The first objection to direct realism was proposed by Russell and it is focused on perceptual variation (how perceptions vary between individuals).
• If direct realism is true, then everybody perceives the exact same physical object, consisting of the same qualities (e.g. size, colour, texture, etc.) as others would perceive it to consist of. This can't be true though, as some people don't perceive certain colours the same way as other people (colour-blind).
• Russell uses the example of a table as claims that two people would perceive the table in a different colour, but the table can't have two colours intrinsic to it.
• Therefore, we don't perceive physical objects directly, but indirectly.
• Therefore, direct realism is false.
• This must also mean that there is a difference between appearance and reality (an appearance-reality distinction).

Objection 1 continued...

• Russell's argument from perceptual variation can be represented using the following argument outline:
• P1. There are variations in perception.
• P2. Our perception varies without any corresponding changes in the physical object we perceive.
• C1. Therefore, the properties that physical objects have and the properties they appear to have are not identical.
• C2. Therefore, what we are immediately aware of in our perception is not the same as what exists within the external world.
• C3. Therefore, direct realism is false.

Response to objection 1

• Direct realism can respond to Russell's colour example by claiming that the two people did not perceive two different colours, but different shades of the same colour.
• Additionally, tests can be carried out to determine the properties of a physical object.
• For example, a table could be tested to see if it was rectangular by trying to fit it through a doorway. If it fits, then the table is rectangular. If not, then the table is not rectangular.
• Direct realism can also append to its theory the idea of relational properties.
• These are properties something has in relation to something else.
• For example, 'being to the north of' is a relational property that Manchester has to London.
• Direct realists do not have to argue that all properties are mind-independent.
• This is because relational properties would be mind-dependent as they depend upon the relationship between our perception of an object and the physical object itself.
• However, the physical objection would still be mind-independent as we can directly perceive it.

Objection 2: Illusion

• There is a difference between how something 'is' and how something 'appears' in an illusion. Illusions are subjectively indistinguishable from veridical perception.
• Subjectively indistinguishable - Can't tell the difference between two or more things.
• Veridical perception - Perception of the way the world actually is.
• Because there is a difference between how something 'is' and how it 'appears' to be in an illusion, there must be an appearance-reality distinction.
• This means that direct realism is wrong as it claims that there is no appearance-reality distinction.
• 
• Caption: The straw appears to both be enlarged and displaced by the water. However, this is surely not what the physical object is actually like, only as it appears. Therefore, in illusions, there must be an appearance-reality distinction. This example is shown in the argument outline.

Objection 2 continued...

• The argument from illusion can be expressed using the following argument outline:
• [Note: For this argument, the 'property F' refers to an object having the property of being crooked]
• P1. We perceive something as having some property F (e.g. the pencil is crooked)
• P2. When we perceive something as having some property F, then there is some property F.
• P3. In an illusion, the physical object does not have the property F (e.g. the pencil is not crooked).
• C1. Therefore, in illusions, what has the property F is mental, a sense-datum.
• C2. Therefore, in illusions, we see sense-data, and not physical objects, immediately.
• P4. Illusions can be 'subjectively indistinguishable' from veridical perception.
• C3. Therefore, we see the same thing, namely sense-data, in both illusions and veridical perception.
• C4. Therefore, in all cases, we see sense-data, and not physical objects, immediately.
• C5. Therefore, direct realism is false.

Response to objection 2

• Direct realism can use the same response for this objection that was used for objection 1 (that is, the response involving relational properties).
• When the pencil is submerged in water, it has the property of looking crooked. Nothing is actually crooked, and so P2 is wrong.
• Property F is a relational property that is mind-dependent.
• In illusions, we perceive the 'looks' properties of a physical object.
• These 'looks' properties do not match the 'is' properties of the pencil, but we still directly perceive the physical object and its properties.

Objection 3: Hallucinations

• The objection from hallucinations is similar to the argument from illusions, with the only difference being that hallucinations do not necessarily involve a physical object.
• As such, the argument outline from hallucinations is rather similar to that of illusions:
• P1. In hallucinations, we perceive something as having some property F.
• P2. When we perceive something as having some property F, then there is some property F.
• P3. In a hallucination, we do not perceive a physical object at all.
• C1. Therefore, what we perceive must be mental -- sense-data.
• P4. Hallucinations can be experiences that are 'subjectively indistinguishable' from veridical perception.
• C3. Therefore, we see the same thing, namely sense-data, in both hallucinations and veridical perception.
• C4. Therefore, in all cases, we see sense-data, and not physical objects, immediately.
• C5. Therefore, direct realism is false.

Response to objection 3

• Direct realists can respond by saying that veridical perception and hallucinations are not from the same mental state.
• Veridical perception comes from our mental state of perception.
• Hallucinations come from our mental state of imagination.
• Therefore, just because we can't tell hallucinations apart from veridical perception when we experience them, it does not mean that they are the same thing.

Objection 4: Time-lag

• We perceive the world using both light and sound waves, but even those take time to travel.
• For example, it takes light waves 8 minutes to travel from the Sun to the Earth.
• This means that when we look at the Sun, we are not seeing it in its current state, but 8 minutes ago.
• Therefore, there isn't a direct perception of the objects around us, as this case would be the same for all objects (light waves would take time to travel from the object to one's retina, despite how small that amount of time would be).

Response to objection 4

• Direct realists can respond to the time-lag argument by saying that there is a difference between 'what' we perceive and 'how' we perceive it.
• Only in some cases, we see the light rays reflecting off of surfaces. In normal conditions, the light waves are part of the perceptual 'story'.
• We still perceive physical objects directly, but in the past.
• For example, we perceive the Sun directly as it was 8 minutes ago.

Indirect realism

• Indirect realism is a realist theory of perception
• This means that it claims the external world (with all of its mind-independent objects and their properties) exist.
• Different from direct realism, indirect realism claims that we do not perceive physical object6s directly but indirectly through our sense data.
• Therefore, according to indirect realism, there is an appearance-reality distinction.
• All of the objections used against direct realism suggest that there is an appearance-reality distinction like indirect realism does.
• This means that all objections to direct realism can be used as arguments to support indirect realism.

Locke on primary and secondary qualities

• Locke claims that there are two types of properties: primary and secondary.
• Primary properties...
• ... are intrinsic to the physical object, meaning that they can't be separated from the physical object without the object itself being changed.
• ... are mind-independent; they do not depend upon one's perceptual relationship with the physical object and so, they do not vary in their perception.
• ... 'resemble' the physical object.
• Examples include SIZE, SHAPE, SOLIDITY, EXTENSION
• Secondary qualities...
• ... are not intrinsic to the physical object and thus, can be separated from the physical object without the object itself being changed.
• ... are mind-dependent; they depend upon the perceptual relationship between the individual and the physical object and thus, they can vary in their perception.
• ... 'resemble' the effects of the primary qualities.
• Examples include COLOUR, TASTE, TEMPERATURE, SOUNDS

Objection 1: Scepticism (and response)

• According to indirect realism, we perceive physical objects indirectly through direct perception of our sense data.
• If we only perceive our sense data directly, how do we know that physical objects exist?
• One may respond to this question with an argument as follows.
• P1. Sense-data is private, and so nobody else can experience my sense data.
• P2. People exist and have their own sense-data.
• P3. Their sense-data is similar to mine.
• C1. Therefore, there must be something that exists (physical objects) which correlates to all of the sense-data.