Ask students to read the following Locke extracts – what’s he driving at, and what are his key examples?
Which leads us to?
Perception 2016 revision 2. indirect realism part 2
The Syllabus: Indirect Realism
• the immediate objects of perception are mind-
dependent objects that are caused by and represent
mind- independent objects.
• Issues, including:
– it leads to scepticism about the ‘existence’ of the external world (attacking
‘realism’) – responses to this (external world is the ‘best hypothesis’
(Russell); coherence of the various senses and lack of choice over our
– it leads to scepticism about the ‘nature’ of the external world (attacking
‘representative’) - responses (sense data tell us of ‘relations’ between
objects (Russell); the distinction between primary and secondary qualities
– problems arising from the view that mind-dependent objects represent
mind-independent objects and are caused by mind-independent objects.
• Indirect realism: the immediate objects of
perception are mind-dependent objects that
are caused by and represent mind-
• Indirect Realists: John Locke, Rene Descartes,
David Hume, Bertrand Russell etc
Problem 2: Can we know the external world as it
really is? Some sceptical thoughts.
• Assume, hypothetically, that the external
world does exist (accept the argument
from best explanation!)
– But what is the world’s nature?
• Key Question: If we know it only through
representations, could there be a gap
between how we see the world and how
it really is?
– Re + Presentation – even the shape of
this word shows there is a gap…
– How representative is our view of the
way the world really is?
• [Worse: solipsism…If sense-data are
private to the individual: could there be a
gap between how the individual sees the
world, and how others see it?
• Might the world of representations have
only one inhabitant?]
How can you check the correctness of
your mental image if you don’t have
direct access to the real?
How can you check the reality of the
cinematic image, if you cannot leave
Responses to this criticism
‘Sense data’ tell us of ‘relations’ between objects – Russell.
‘We agreed provisionally that physical objects cannot be quite like our sense-
data, but may be regarded as causing our sensations. … we may assume that
there is a physical space in which physical objects have spatial relations
corresponding to those which the corresponding sense-data have in our
private spaces…we cannot have that immediate acquaintance with physical
distances that we have with distances in our private spaces, or with colours or
sounds or other sense-data. …although the relations of physical objects have
all sorts of knowable properties, derived from their correspondence with the
relations of sense-data, the physical objects themselves remain unknown in
their intrinsic nature, so far at least as can be discovered by means of the
senses. [Still, the] most natural, though not ultimately the most defensible,
hypothesis to adopt in the first instance…would be that, though physical
objects cannot, for the reasons we have been considering, be exactly like
sense-data, yet they may be more or less like’
• How satisfactory is Russell’s reply: ‘we can’t know things as they are, but
only the relations between them?’
• Is this an experiential answer, or a logician’s?
Russell’s Argument in more detail
• His argument: given the argument from perceptual variation, it is clear that physical
space (the space of science) and apparent space (space as we experience it) are not the
• But they are connected in a detailed, systematic, and predictable way.
– Objects cause sense-data, so our bodies are causally affected by objects.
– The relative positions of objects in physical space correspond to the relative
positions of sense-data in apparent space. (Block 3 looks further away than Block
1; it takes us longer to walk there, therefore)
– We can’t know physical objects in themselves, but we can know that the
relationships between sense-data and (assumed, underlying) physical objects must
• The same argument works for time:
– physical time and apparent time are not experientially the same (ever been in a
lesson that dragged out?)
– But even when time-sense varies between perceivers, events occur in the same
• The same argument works for colour, and all other qualities.
– If two objects look the same under the same viewing conditions, then they have
something in common. If they look different, they do not. If two objects feel,
smell, taste the same…
– We can’t know what it is about the physical objects in themselves that enables
these relationships. But we can know that the relationships exist.
Issues with Russell’s response
• But: is it satisfactory to reduce our knowledge of the world to knowledge of sense-data,
and knowledge of the relationships between sense-data?
• How much of an advance is it to claim that sense-data tell us only about real relations
between objects, objects which we cannot experience directly?
• We may be able to hypothesise a ‘real’ physical space, but how are we helped by this
notion if we cannot experience this space ourselves?
• How can we be confident that our sense-data are ‘more or less like’ the underlying
objects that produce them? (Russell admits that his assertion that they are is not very
• He seems to admit that at best it only seems that we perceive mind-independent
– This is counter-intuitive: try describing what you experience in terms of pure sense-
data without bringing in physical object terms…
– Are physical object terms merely ‘shorthand’ for collections of sense-data?
Task: Describe an everyday physical object using only sense-data terms.
Locke’s Notion of Primary and Secondary
• Locke argues that his distinction between primary and
secondary qualities explains the nature of the external world.
• Text source: Essay Concerning Human Understanding, Book 2
chs. 8 and 21
• Our encounters with material bodies (i.e. physical objects)
leads to their producing various ideas in us.
– E.g. Our experience of a particular Granny Smith apple produces ideas
of a certain roundish shape, a certain size, a certain range of green
shades, a certain tart taste, and certain crisp texture etc…
• Locke distinguishes the qualities (=properties) of the material
objects from the ideas (of those qualities) in our mind.
– Qualities are in the external objects.
– Ideas (of qualities) are in the mind.
Locke on Qualities and Ideas (of Qualities)
taken from his ‘An Essay Concerning Human Understanding’ (1690)
8. [Ideas and Qualities] Whatsoever the mind perceives
in itself, or is the immediate object of perception,
thought, or understanding, that I call idea; and the power
to produce any idea in our mind, I call a quality of the
subject wherein that power is. Thus a snowball having the
power to produce in us the ideas of white, cold, and
round, as they are in the snowball, I call qualities; and as
they are sensations or perceptions in our understandings,
I call them ideas;
Explain Locke’s distinction between ideas and qualities.
(shape, size, motion, solidity)
– ‘Motion’ = the states of moving or being at rest
– ‘Solidity’ = occupies 3D space in some manner (modern quantum physics has
redefined this property somewhat)
• Primary qualities are intrinsic qualities in bodies: physical objects must
have these qualities.
• The idea of a physical object always includes its being a size, a shape,
being in rest or motion, occupying space.
• These qualities are mind-independent and objective.
• And: our ideas of these primary qualities resemble the qualities
(colors, odors, tastes, textures, sounds)
• These qualities are perceiver-dependent and therefore subjective.
• Physical objects can be conceived of without them: there can be
e.g. odourless, intangible (=without feel), transparent (=without
• Secondary qualities are in material bodies only as causal powers
(resulting from the PQs) to cause certain sorts of sensation in minds
– These causal powers result from the PQs: a particular microstructure of PQs
interacts with our senses to produce such-and-such a sensation in us.
• So the secondary qualities are really nothing in the object beyond
the primary qualities and so our ideas of these secondary qualities
in no way resemble the qualities themselves.
Primary versus Secondary
• Primary qualities are objective and invariant.
– They do not depend on human perception.
– A being who lacks vision could understand e.g. a square
– Bernard Williams: ‘”What is real is accessible from any point of
• Secondary properties such as colours, sounds are not real
properties of objects, but are mutable and subjective.
– They do depend on human perception.
– What we perceive of as colours, noises, smells would not be
perceived as such by other creatures (bats!)
• Yet primary qualities cause secondary ones.
• So our mutable sensory experience rests on something wholly
How do secondary qualities
relate to primary ones?
• caused by, reducible to primary qualities (movement, shape,
• so perceiver-dependent qualities are caused by perceiver-
• so (assumption) they are good representations of (=
systematically correlated with) the world of primary qualities.
Scientific account of sound: sound is
movement of air molecules
Scientific account of taste: taste is
shape of scent molecules
Locke’s distinction between
Primary and Secondary Qualities
9. [Primary qualities] Some qualities…are utterly [constant and]
inseparable from the body, in whatever state it be, [regardless of] all the
alterations and changes it suffers…e.g. Take a grain of wheat, divide it into
two parts; each part has still solidity, extension, figure, and mobility [or
rest and motion]: divide it again, and it retains still the same qualities [and
so on, regardless of how small it becomes]… these I call original or
primary qualities of body
10. [Secondary qualities of bodies] Secondly, such qualities which in
truth are nothing in the objects themselves but power to produce various
sensations in us by their primary qualities, i.e. by the bulk, figure, texture,
and motion of their insensible parts, as colours, sounds, tastes, &c. These
I call secondary qualities. …
11. [How bodies produce ideas in us] Bodies produce ideas in
us…[primary qualities cause external] motions…continued by our nerves,
or animal spirits…to the brains or the seat of sensation, there to produce
in our minds the particular ideas we have of them…
Locke: only primary qualities are
17. [Only primary qualities exist independently of being
perceived] The particular bulk, number, figure, and
motion of the parts of fire or snow are really in them,—
whether any one’s senses perceive them or no: and
therefore they may be called real qualities…But light,
heat, whiteness, or coldness are not…Take away the
sensation of them; let not the eyes see light or colours,
nor the ears hear sounds; let the palate not taste, nor
the nose smell, and all colours, tastes, odours, and
sounds, as they are such particular ideas, vanish and
cease, and are reduced to their causes, i.e. bulk, figure,
and motion of parts.
Locke: primary qualities cause secondary ones.
18. [Secondary qualities exist in things only because
of primary qualities] Let us consider the red and
white colours in porphyry [purple stone with white
flecks]. Hinder light from striking on it, and its colours
vanish; it no longer produces any such ideas in us:
upon the return of light it produces these
appearances on us again. Can any one think any real
alterations are made in the porphyry by the presence
or absence of light… it is plain it has no colour in the
dark…It has, indeed, such a configuration of particles,
both night and day, as are apt, by the rays of light
rebounding from some parts of that hard stone, to
produce in us the idea of redness, and from others
the idea of whiteness; but whiteness or redness are
not in it at any time, but such a texture that hath the
power to produce such a sensation in us.
Locke: primary qualities are always present,
secondary qualities are highly mutable.
20. Pound an almond, and
the clear white colour will be
altered into a dirty one, and
the sweet taste into an oily
one. What real alteration can
the beating of the pestle
make in any body, but an
alteration of the texture of
Locke: Primary Qualities are True Resemblances
15. [Ideas of primary qualities are true
resemblances – but secondary qualities are
not]…the ideas of primary qualities of bodies are
resemblances of them, and…do really exist in the
bodies themselves, but the ideas produced in us by
these secondary qualities have no resemblance to
the bodies themselves. They are, in the bodies we
denominate from them, only a power to produce
those sensations in us: and what is sweet, blue, or
warm in idea, is but the certain bulk, figure, and
motion of the insensible parts…
Locke: perceptual variation explained
16 Examples. Flame is denominated hot and light;
snow, white and cold; and manna [sugar from plant
sap] white and sweet...And yet…the same fire that,
at one distance produces in us the sensation of
warmth, does, at a nearer approach, produce in us
the far different sensation of pain. Is this idea of
warmth actually in the fire or is this idea of pain in
the fire? The fire produces these ideas only
through the bulk, figure, number, and motion of its
Writing task: Primary Versus Secondary
22. …I hope I shall be pardoned this little excursion into
science. But now I have distinguished the primary and
real qualities of bodies, which are always in them (viz.
solidity, extension, figure, number, and motion, or rest…)
from those secondary and imputed qualities, which are
but the powers of several combinations of those primary
ones. [So now] we know which ideas are, and which are
not, resemblances of something really existing.
Explain the difference between primary and secondary
qualities, and the relationship between them.
Arguments for the primary/secondary distinction 1
Locke’s view protects the objectivity of the external world and explains
• Primary qualities capture those features of experience that provide
us with a conception of an objective world, distinct from the merely
subjective series of observer-relative sensations.
• We are subject to illusion and perceptual variation with respect to
secondary qualities, but not with primary qualities. Locke’s
example: a cold hand and a hot hand dunked into the same water
feel subjectively different sensations.
• Temperature is motion, so the primary qualities of the water are invariant.
• The same primary qualities cause different secondary qualities because of
their interaction with the perceiver.
• Locke’s account has both objective reality and also saves the
appearances we perceive.
• Locke’s claim that more basic physical qualities cause perceiver-
dependent qualities maps neatly to the emerging scientific world-
• Science, then as now, distinguishes explanatorily basic primary
qualities from less fundamental secondary qualities.
• C17 ‘corpuscularian hypothesis’ posited primary qualities as characterizing the
fundamental nature of matter.
• The hard physical sciences these days deal only with the measurable physical
qualities of objects, the primary qualities
• present-day science has incredible explanatory power because of the
prevalence of the ‘atomic model’ of matter, even if we have revised our notion
• Science also causally explains what it actually means for objects to
have secondary properties, and does so in terms solely of primary
Arguments for the
primary/secondary distinction 2
Locke’s view fits neatly with the scientific worldview
Issues with Locke’s account 1
Locke’s picture of secondary qualities seems inconsistent.
• He claims that secondary properties are qualities in objects that
cause ideas in our minds.
• These qualities are relational properties that arise in our minds
because of certain arrangements of primary qualities. They exist
outside our minds.
• Yet we cannot think of e.g. colour as ‘in’ objects (the porphyry
example shows that colour is not in the stone itself)
• Secondary properties such as colours are in fact effects on us.
• So secondary qualities, as effects, are in our minds only. They are
ideas. (And variable ones: think of perceptual variation…)
• But Locke has, earlier, argued that ideas (=effects of qualities) and
qualities (=causes of ideas) aren’t the same.
• Are secondary qualities ideas or qualities? Locke’s account could be
said to blur the distinction…
Issues with Locke’s account 2
Primary qualities are not distinguishable from secondary ones
• Locke claims that
– mind-independent qualities cause mind-dependent qualities,
– but we perceive both kinds of qualities simultaneously.
• Yet how do we perceive primary qualities such as shape,
motion, solidity and size?
– Could we not argue (Berkeley!) that these mind-independent
qualities are in fact only perceived through secondary
qualities?(shape, motion etc are derived from colour etc)
– Hence, if mind-independent qualities can only be known
through mind-dependent ones, their existence and their
correct resemblance to underlying physical objects cannot
– Locke’s account merely buries issues of existence and
resemblance – it does not explain them.
Last bit of Indirect Realism syllabus
There are problems arising
from the view that mind-
objects and are caused by