Hume’s scepticism
Michael Lacewing
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© Michael Lacewing
Hume’s fork
• We can only have
knowledge of
– Relations of ideas
– Matters of fact
• Relations of ideas are a
priori and analytic
• Matters of fact are a
posteriori and synthetic
A priori knowledge
• A priori: knowledge that does not
require (sense) experience to be
known to be true (v. a posteriori)
• It is not a claim that no experience was
necessary to arrive at the claim, but
that none is needed to prove it.
Analytic/synthetic
• A proposition is analytic if it is true or false
just in virtue of the meanings of the words.
• A proposition is synthetic if it is not analytic,
i.e. it is true or false not just in virtue of the
meanings of the words, but in virtue of the
way the world is.
Knowledge of matters of
fact
• Knowledge of matters of fact is always
a posteriori and synthetic.
• We gain it by using observation and
employing induction and reasoning
about probability.
• The foundation of this knowledge is
what we experience here and now, or
can remember.
Causal inference
• All our knowledge
that goes beyond
what is present to
our senses or
memory rests on
causal inference.
Hume’s challenge
• Why and how do we move from ‘this is what
happens here and now’ to ‘this is what
happens generally’?
• Similar causes have similar effects, and so
the future will be like the past and events
elsewhere that we haven’t actually
experienced will be like events we have
experienced.
• How do we know this?
Past and future
• Because in the past, similar
causes had similar effects.
• But how do we know similar
causes will have similar effects
in the future?
• Past experience can give me
‘direct and certain information
of those precise objects only,
and that precise period of time,
which fall under its cognizance’.
Hume’s solution
• We draw the inference from cause to effect
without reasoning or argument, but on the
basis of a principle of the ‘imagination’ custom - that has bound the two ideas – of
the cause and of the effect – together in our
minds.
• Custom is a natural instinct of the mind, a
disposition we simply have in the face of
experience of constant conjunction.
Expectation
Potassium in water (usual) - Potassium in water (unusual)
• When we experience something that has been a
cause in the past (one billiard ball striking another),
we immediately believe that its usual effect is
about to occur.
Scepticism
• Without custom, we would be unable to
draw causal inferences.
• Without causal inference, we have no
knowledge of anything beyond what was
present to our senses and memory.
• But custom is not reason. So we have no
reason to believe that what we haven’t
experienced will be like what we have.
External world
1. We are naturally disposed to believe in the
external world, and at first we think that
our impressions perfectly resemble it.
2. On reflection, we don’t suppose a table
gets smaller as we move away.
3. So we must accept that what is
immediately available to the mind is only
ideas, which don’t resemble objects
perfectly; yet we continue to think that
the objects represented persist
independently of our impressions.
External world
4. But now we must wonder how we can show
that our impressions must be caused by
such independent objects!
5. Experience can’t show this, because all
that experience has available is the
impressions themselves, not the connexion
between impressions and objects.
6. The belief in the external world,
therefore, is groundless.
Was Hume a sceptic?
• Hume clearly claimed we do have knowledge
as a result of causal inference, and since
knowledge of the external world is a
prerequisite for this, we have knowledge of
that, too.
• Hume has two models of ‘knowledge’:
– Practical: knowledge rests on custom and other
instincts, such as belief in the external world.
– Reason only (deduction): Knowledge only of what
is immediately present to our senses, and what
we can remember.