Overview – Knowledge from Perception

This A level philosophy topic looks at 3 theories of perception that explain how we can acquire knowledge from experience, i.e. a posteriori. They are:

The theories disagree over such issues as whether the external world exists (realism vs. anti-realism) and the way we perceive it (direct vs. indirect). Each theory also has various arguments for and against. These key points are summarised below:

Direct Realism Indirect Realism Idealism
External world Mind-independent Mind-independent Mind-dependent
Perception method Direct Indirect (via sense data) Direct
  • Locke
  • Russell
  • Berkeley

Direct Realism

Direct realism is the view that:

  • The external world exists independently of the mind (hence, realism)
  • And we perceive the external world directly (hence, direct)

So, basically, what you see is what you get.

When you look at, and perceive, a tree, you are directly perceiving a tree that exists ‘out there’ in the world. You are also perceiving its properties (size, shape, smell, etc.).

So, the immediate objects of perception are mind-independent objects and their properties.

Direct Realism

Direct realism is often thought of as the common sense theory of perception. When asked what you see, you describe the external object itself, not your perception of it. For example:

“What do you see?”

“A table.”

You wouldn’t respond with:

“Brown patches of sense data in a rectangular arrangement.”

At least on the face of it, perceptual experience presents itself to us as mind-independent objects. However, there are a number of issues with this simple explanation.

Problems for direct realism

Bertrand Russell: Argument from perceptual variation

The shape of the table looks different depending on the angle you are perceiving it from

Differences in perceptual variation provide a problem for direct realism.

For example, when I stand on one side of the room, a shiny wooden table may have a white spot where the light is shining on it. But to someone standing on the other side of the room, there may be no white spot.

But the white spot is either there or it isn’t – it can’t be both! So, at least one of us is not perceiving the table directly as it is.

Russell also talks about the shape of a table. From directly overhead, it may appear to be rectangular. But from a few metres away it may look kite-shaped. Again, it can’t be both shapes!

These examples highlight differences in our perception of the table and the table itself. However, according to direct realism, there should be no such differences between perception and reality.

Possible response:

Direct realism can respond by refining the theory and introducing the idea of relational properties.

A relational property is one that varies in relation to something else. For example, being to the left or right of something (e.g. ‘the cupboard to the left of the fridge’) is a real property that something can have – but it varies relative to other objects. Similarly, we could say that appearing kite-shaped is a real property a table can have relative to certain perceivers.

In other words: The object itself does not change, but the perceiver does – and thus the relational properties of the object change.

Argument from illusion

Remember, direct realism says that we perceive the external world directly as it is.

But if this is true, how is it that reality (i.e. the external world) can be different to our perception of it?

For example, when a pencil is placed in a glass of water, it can look crooked. But it isn’t really crooked.

If direct realism is true, the external world would be exactly as we perceive it. However, in the case of illusions, there is an obvious difference between our perception and reality.

Possible response:

Similar to the response to perceptual variation, the direct realist could reply that the pencil has the relational property of looking crooked to certain perceivers (even though it isn’t really crooked). However, this response fails to explain the argument from hallucination.

Argument from hallucination

This is a more extreme version of the argument from illusion.

Direct realism says that when we perceive something, we are perceiving something in the external world (directly).

But during hallucinations – perhaps as a result of being ill or taking drugs – we perceive things that aren’t even there.

So what is causing these perceptions? It can’t be the external world – at least not directly – because there is no external object being perceived at all!

Time lag argument

The sun is 149,600,000 km from earth.

Light travels at 299,792,458 metres per second.

This means it takes approximately 8 minutes for light to reach earth.

So, when you look at the sun, you are seeing it as it was 8 minutes ago – i.e. there is a difference between the sun itself and your perception of it. In other words, you are not perceiving the sun directly.

Possible response:

The direct realist can argue that this response confuses what we perceive with how we perceive it. Yes, we perceive objects via light and sound waves and, yes, it takes time for these light and sound waves to travel through space. But what we are perceiving is still a mind-independent object – it’s just we are perceiving it as it was moments ago rather than how it is now.

direct realist response to time lag objection

Indirect realism

Indirect realism is the view that:

  • The external world exists independently of the mind (hence, realism)
  • But we perceive the external world indirectly, via sense data (hence, indirect)

Indirect realism says the immediate object of perception is sense data. This sense data is caused by, and represents, the mind-independent external world.

Indirect Realism theory of perception

Most philosophers who argue for indirect realism start with the problems for direct realism. The many differences between reality and our perception of it, they say, make direct realism implausible.

Instead, they claim that what we really perceive is sense data.

Sense data

Sense data can be described as the content of perceptual experience.

It’s not a physical thing, it exists in the mind. However, sense data is said to be caused by and represent mind-independent physical objects (see diagram above).

Sense data is private. No one else can experience your sense data.

This avoids the problems with direct realism described above. For example, differences in perceptual variation can be explained by differences in sense data. The object itself stays the same throughout even if the sense data changes.

John Locke: primary and secondary qualities

An idea closely related to sense data is John Locke’s distinction between primary and secondary qualities:

Primary qualities Secondary qualities
Properties inherent in the object itself Powers of an object to cause sensations in humans
Objective Subjective
  • Size
  • Shape
  • Motion
  • Number
  • Colour
  • Taste
  • Smell
  • Feel

Locke uses various examples to illustrate this distinction.

One such example is porphyry – a red and white stone. Locke says that when you prevent light from reaching porphyry, “its colours vanish”. However, the primary qualities – size, shape, etc. – remain.

The distinction between primary and secondary qualities can be used to support indirect realism. Like sense data, this distinction explains the difference between reality (primary qualities) and our perception of it (secondary qualities).

For a problem with this view, see Berkeley’s criticism of the distinction between primary and secondary qualities.

Problem: scepticism

A problem for indirect realism is that it leads to scepticism about the nature and existence of the external world.

veil of perception indirect realism

Look at the two diagrams above. What would be the difference from the perceiver’s perspective between the two? What difference would it make to the perceiver if there was no physical world at all?

The answer, surely, is nothing.

If we only perceive sense data, and not the object itself, how can we know anything about the external world? There is no way of telling if the sense data is an accurate representation of the external world – or even that there is an external world at all!

We can’t get beyond the veil of perception (sense data) to access the external world behind it. So, what grounds does the indirect realist to support their claim that there is a mind-independent external world the causes sense data?

Indirect realist replies to scepticism

Russell’s reply

Bertrand Russell, an indirect realist, concedes that there is no way we can conclusively defeat this sceptical argument. However, he argues that the existence of a mind-independent external world is the best explanation for sense data.

Locke’s first reply

John Locke offers two responses to the sceptical challenge.

First, Locke notes how he is unable to avoid having certain sense data produced in his mind when he looks at an object. By contrast, memory and imagination allows him to choose what he experiences. Locke concludes from this that whatever causes his perceptions must be something external to his mind as he is unable to control these perceptions.

Possible response:

However, even if Locke succeeds in proving something external, he doesn’t succeed in proving that sense data is in any way an accurate representation of the external world. The sceptic could argue that the external world may be completely different to our perception of it and there’s no way we could know.

Locke’s second reply

Second, Locke argues that the different senses confirm the information of one another. For example, you can write something on a piece of paper and see the words. Then, you can get someone to read the words out loud and thus hear the same information via a different source.

Possible response:

But does this really succeed in defeating the sceptical challenge? The information you hear may be equally misrepresentative of the external world as the information you see.


Idealism is the view that:

  • There is no external world independent of minds (so it can be labelled an anti-realist theory)
  • We perceive ideas directly

In other words, the immediate objects of perception are mind-dependent ideas.

Idealism theory of perception

Unlike direct realism and indirect realism, idealism says there is no mind-independent external world. Hence, idealism can be classed as an anti-realist theory.

Instead, idealism claims that all that exists are ideas.

What’s more, idealism says that unless something is being perceived, it doesn’t exist!

Bishop Berkeley: Three Dialogues Between Hylas and Philonous

Bishop George Berkeley (1685-1753) is the most famous proponent of idealism.

Berkeley offers various arguments against the existence of a mind-independent external world. These arguments include a variation of the sceptical argument described earlier. Berkeley asks how – if realism is true – we can link up our perception with the objects behind it. Again, it seems we can’t look past the veil of perception.

Berkeley then goes on to challenge against Locke’s primary and secondary quality distinction, arguing that so-called primary qualities are equally mind-dependent as primary qualities.

He later gives his master argument: An argument that the very idea of mind-independent objects is inconceivable and impossible.

Attack on Locke’s primary/secondary quality distinction

Berkeley argues that the only thing our senses perceive are qualities, and nothing more. For example, we perceive colours and shapes via vision, sounds via hearing, flavours via taste, and so on.

But we never perceive anything in addition to these qualities.

This claim forms the basis of one of Berkeley’s arguments against the existence of a mind-independent external world.

Earlier, we looked at Locke’s distinction between primary and secondary qualities. Recall that Locke said primary qualities are inherent in the object, whereas secondary qualities are not:

“Take away the sensation […] and all colours, tastes, odours, and sounds […] vanish and cease, and are reduced to their causes”

Locke – An Essay Concerning Human Understanding (Bk. 2, Ch. 8, § 17)

Based on this extract, Locke seems to be saying that secondary qualities are mind-dependent.

Berkeley himself provides his own arguments that secondary qualities are mind dependent. For example, heat (a secondary quality) can be experienced as pain (mind-dependent). When you burn your hand on a hot fire, you don’t feel the pain and the heat separately. You feel one sensation: painful heat.

So, it seems Berkeley and Locke are in agreement so far: secondary qualities are mind dependent.

Where Berkeley disagrees with Locke is the status of primary qualities. Locke says they’re mind-independent, but Berkeley argues they’re not.

Berkeley offers various versions of perceptual variation to support the claim that primary qualities depend on the mind just as much as secondary qualities do:

  • Something that looks small to me may seem large to a small animal
  • Something that looks small at a distance may seem large close up
  • A smooth surface may look jagged under a microscope
  • An object that appears to be moving quickly to humans may appear to be moving slowly to a fly

Size, shape, and motion: they are all primary qualities but these examples show how our perception of them differs depending on the circumstances.

So, Berkeley argues, we can’t say these objects have one single size, shape, or motion independent of how it is perceived. So, primary qualities are also mind-dependent.

Bring together Berkeley’s points and we get the following argument:

  • When we perceive an object, we don’t perceive anything in addition to its primary and secondary qualities
  • So, everything we perceive is either a primary quality or a secondary quality
  • Secondary qualities are mind-dependent
  • Primary qualities are also mind-dependent
  • Therefore, everything we perceive is mind-dependent

So, Berkeley uses Locke’s primary and secondary quality distinction to prove that everything we perceive is mind-dependent. This implies that there is no such thing as a mind-independent external world (and so realism is false).

The master argument

The second of Berkeley’s arguments for idealism/arguments against indirect realism is known as the master argument.

The dialogue (between Hylas and Philonous) for the master argument can be summarised as:

  • P: Try to think of an object that exists independently of being perceived.
  • H: OK, I am thinking of a tree that is not being perceived by anyone.
  • P: But that’s impossible! You might be imagining a tree in a solitary place with no one perceiving it – but you’re still thinking about the tree. You can think of the idea of a tree, but not of a tree that exists independently of the mind.

So, Berkeley’s master argument is essentially that we cannot even conceive of a mind-independent object because as soon as we conceive of such an object, it becomes mind-dependent. Thus, mind-independent objects are impossible.

Possible response:Berkeley's master argument for idealism

However, the conclusion does not necessarily follow: just because it’s impossible to have an idea of a mind-independent object, it doesn’t mean that mind-independent objects are themselves impossible.

God as the cause of perceptions

Despite arguing against the existence of a mind-independent external world, idealism does not lead to the same veil of perception problem that indirect realism does. The veil of perception disappears when we realise that the meaning of words like ‘physical object’ refer to bundles of ideas and not mind-independent objects (as realism assumes). By perceiving ideas, we are perceiving reality. That’s what reality is: ideas.

This still leaves the question of what causes these ideas. Berkeley’s answer is God, and his argument is this:

  • Everything we perceive is mind-dependent (as argued above)
  • There are 3 possible causes of these perceptions:
    1. Ideas
    2. My own mind
    3. Another mind
  • It can’t be ideas (1), because ideas by themselves don’t cause anything
  • It can’t be my own mind (2), because if I was the cause of my own perception then I’d be able to control what I perceived
  • Therefore, the cause of my perception must be 3: another mind
  • Given the complexity, variety, order, and manner of my perceptions, this other mind must be God

The role of God here resolves an obvious objection to idealism: If Berkeley’s theory is true, then presumably my desk and everything in my office no longer exist when I leave the room and stop perceiving them. So, why are they still there when I get back? Berkeley’s response is that the office (and all ‘physical objects’) constantly exist in the mind of God.

What we perceive, Berkeley says, are copies of ideas that exist eternally in God’s mind (when He wills me to perceive them). This resolves another potential criticism: We might object that when you and I both look at the same tree, we are perceiving different things – i.e. two separate ideas. However, since we are both perceiving the same copy of God’s idea, we both can be said to be perceiving the same thing.

Problems for idealism


Solipsism is the view that one’s mind is the only mind that exists.

And Berkeley’s earlier argument – that everything one perceives is mind-dependent – suggests that there is no reason to believe anything exists beyond one’s experience.

I never perceive other minds, all I perceive are ideas – so what reason do I have to believe that other minds exist at all?

Hallucination and illusion (again)

As a direct theory of perception, idealism makes no distinction between appearance (perception) and reality.

But this makes it difficult to explain the argument from illusion that is also a problem for direct realism. For example, why does a pencil in water look crooked when it isn’t?

Berkeley’s answer is that to say “the pencil is crooked” is to say that the pencil would look crooked under normal conditions. Since this is obviously false, we should say “the pencil looks crooked” to avoid this implication.

But what about hallucination? If, as Berkeley contends, “to be is to be perceived” – are we to say that hallucinations are just as real as ordinary perception? Also, why would God cause such perceptions?

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