Perception as a source of knowledge

• Direct realism is the epistemological theory that the immediate objects of perception are mind-independent objects and their properties.
• There are no additional objects between the perceiver and the perceived objects, like sense-data.
• Sense data are mental entities that mediate between sensory stimuli and perceptions.
• The argument from perceptual variation shows that there are variations in perception without changes in the object. When a table is viewed from different angles, it appears to change shape. So properties of objects and properties objects appear to have are not identical. Our immediate perception is not identical to what exists mind-independently, so we do not perceive objects directly, rather via sense-data.
• In response to perceptual variation, direct realists introduced relational properties. These are properties of an object in relation to something else. For example, 'being to the north of' something is a relational property. The table has the property of looking irregularly shaped in relation to the perceiver, but being and appearing irregularly shaped are distinctly different.
• An illusion is a misrepresentation of a sensory stimulus. When a pencil is submerged in water, it appears to become crooked. But, when taken out, the pencil does not possess this property. So, the pencil appearing crooked must be something mental, like sense-data.
• In response to illusions, we can argue that relational properties solve the problem. The pencil only has the property of looking crooked, rather than actually being crooked. Properties of being crooked and looking crooked are distinctly different, so we can say that in illusions we perceive the properties of how objects look, but we still perceive objects directly.
• The argument from hallucination shows that we see sense-data in all perception, rather than perceiving objects directly. Hallucinations are sensory experiences which do not exist in the external world. When we perceive something, we are actually perceiving nothing. It must be mental. These experiences are subjectively indistinguishable from veridical perception, so we see sense-data in all cases of perception.
• In response to hallucinations, direct realists use the disjunctive theory of perception to show that hallucinations and veridical perception are two different types of mental state. Perception is a form of cognitive contact with the world, but in hallucination, the person has no connection with the world. The fact that they are subjectively indistinguishable from veridical perception tells us nothing important about perception. We are imagining during hallucination, not perceiving.
• The argument from time-lag argues that what we perceive is the physical medium by which we detect physical objects, rather than the physical objects themselves. For example, sunlight takes around 8 minutes to reach Earth. If the sun disappeared, we would not notice until 8 minutes later, even though it no longer exists. So, we do not perceive objects directly, just the lightwaves.
• In response to time-lag, direct realists argue that physical mediums are just part of the perception process. Without light waves, it would be impossible to see at all. So, we directly perceive objects, just as they were a moment ago
• Indirect realism is the theory which claims that the immediate objects of perception are mind-dependent sense-data, which are caused by and represent mind-independent objects.
• Sense-data are the contents of our perceptual experiences. They are private and subjective.
• A quality is a power that a physical object has to produce an idea in our mind.
• Primary qualities are utterly inseparable from the object. For example, qualities such as shape, number and solidity.
• Secondary qualities are nothing but powers to produce a sensation. For example, colour, sound and taste. These are related to the perceiver.
• Our experiences of primary qualities resemble the physical object, whereas our experiences of secondary qualities do not. The world as we experience it through our senses compared to the way the world is in itself, are different. So we perceive the world indirectly, via sense-data.
• Indirect realism raises a problem: how can we know that physical objects exist 'behind' our sense-data? To know that objects cause our sense-data, we have to know that physical objects exist. But we can only know about objects via sense-data.
• Russell responds to the problem of scepticism by arguing that the existence of physical objects is the best hypothesis. Either objects exist and cause my sense-data, or they do not. I cannot prove this to be true or false, so I must treat it as a hypothesis. The first hypothesis is better; it appeals to Ockham's razor, and explains why our sense-data works the way it does. For example, if I perceive a cat at position A, and later position B, it is intuitive to say that the cat exists as a physical object, rather than saying it springs into existence.
• Locke's involuntary nature of perception shows that we cannot avoid experiencing certain sense-data. If our eyes are open, we will see our surroundings. By contrast, when imagining, I can choose what I perceive. So, perceptual experience must be caused by something external.
• Locke and Cockburn also respond to the problem of scepticism by arguing that the coherence of the senses is a good reason to believe in the existence of physical objects. Seeing a fire can be confirmed by reaching out to touch it and feeling the heat. One and the same object cause perceptual experiences through different senses. We can accurately infer from one experience to another.
• Weakness for indirect realism: if indirect realism is correct, and the existence of physical objects is just a hypothesis, this shows a weakness in the theory. There may be another better hypothesis that we do to yet know of. It is also counter-intuitive to think that perception doesn't put us in direct contact with physical objects.
• Indirect realism maintains that sense-data represent mind-independent objects. Locke says that objects resemble our sense-data. But, how do we know that physical objects have primary qualities that resemble our experience of them? We can't tell what a cause is like just from its effects.
• Berkeley's argument that mind-independent objects cannot be like mind-dependent ideas asks the question: how can our sense-data, which are fleeting and variable, be like a physical object, which is fixed and constant? How can squareness resemble the idea of squareness? There is nothing like a perceptual experience except another perceptual experience. So, we cannot know the nature of mind-independent objects.
• Idealism is the theory that the immediate objects of perception are mind-dependent objects.
• The main aim of Berkeley's philosophy is to show that there is no such thing as matter or substance existing independently of minds. Everything that exists is either mental or dependent on the mind.
• Berkeley's attack on primary/ secondary quality distinction aims to show that primary qualities vary just as much as secondary qualities. Using perceptual variation, an object which looks small to me will look huge to an ant; a circular straight on looks circular, but from an angle looks elliptical, etc. We cannot say that any one object has a set size or motion. So everything we perceive is mind-dependent.
• Berkeley's master argument argues that we cannot perceive something unconceived or unperceived of. In 3 dialogues between Hylas and Philonous, he asks Hylas to think of an object existing independently of the mind. Hylas claims he is thinking of a tree existing unperceived. But, Berkeley argues that what he is thinking depends on his mind.

  We cannot think of a tree that is neither perceived nor conceived of. We can think of the idea of a tree, but not one existing mind-independently.
• Berkeley argues that God is the cause of our ideas. There are 3 possible causes of our ideas: my mind, ideas, or another mind. Ideas do not cause things, if it were my mind I would be able to control my experiences, so it must be another mind. Given the complexity and systematicity of our ideas, it must be God
• In objection to idealism, we can ask how it tackles illusions and hallucinations. If idealism were to be correct, there must be an idea that corresponds to both. But, we don't want to say that the physical object is as it looks in the illusion. For example, with a pencil submerged in water, idealism would say that the pencil is crooked.
• Hallucinations are also a problem for idealism. Hallucinations are a product of imagination, which is usually voluntary. But, hallucinations are involuntary, like our perceptions. So how can idealism distinguish between the two?
• In response to hallucinations, Berkeley argues that hallucinations are 'dim, irregular and confused'. Even if they were as clear as normal perception, they are not coherently connected with the rest of our perceptual experience.
• Solipsism is the view that I am the only mind which exists.
• Idealism leads to solipsism. If everything I perceive is mind-dependent, how can I know other physical objects (minds) exist?
• In response to the issue that idealism leads to solipsism, Berkeley argues that the mind perceives actively, whereas ideas are passive. So, I am not my ideas, but a mind. I have a notion of what a mind is, because I am one. So, other minds can exist. My perceptions don't originate in my mind, so it must be from another mind (God)
• There are problems raised for the role God plays in idealism. Berkeley claims that our ideas exist in God's mind. But, God cannot have the same perceptual experiences as us, like feeling pain. The objects of our perception undergo change, but God is eternal and unchanging. Also, ideas exist in my mind, not God's.
• In response to the issues raised about God in idealism, Berkeley claims that our ideas are a copy of the ideas in God's mind. Our ideas do not exist as perceptions in God's mind, but as part of his understanding. Although he cannot feel pain, he knows what it is for us to have this experience. What I perceive is what God wills me to perceive.