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Descartes on Certainty (and
Michael Lacewing
[email protected]
© Michael Lacewing
Hyperbolic doubt
• To find the truth,
need to avoid
believing error - so
only believe what is
‘indubitable and
• What is indubitable
appears to be what
we are unable to
What is certainty?
• Subjective, psychological feeling of
• Logical, e.g. a proposition that must
logically be true is certain
• Combined? A proposition that cannot
be doubted by a rational thinker
– Sometimes, we can’t doubt something
because it must be true
Into doubt
• We can be deceived by our senses.
• There are ‘no certain indications by which
we may clearly distinguish wakefulness from
The cogito
• I am certain that I think, I exist. Doubting is
a kind of thinking, and if I were to doubt
that I existed, that would prove I do exist.
• ‘In this first item of knowledge there is
simply a clear and distinct perception of
what I am asserting’. While thinking it, I
cannot doubt it.
‘Clear and distinct’ ideas
• Descartes comes to argue that he can know
what is ‘clear and distinct’. Making this
judgment requires great care, i.e. we can make
• To be clear, an idea must be ‘open and present
to the attending mind’; to be distinct, it must
not only be clear, but precise and separated
from other ideas, so that it ‘plainly contains in
itself nothing other than what is clear’
(Principles I.45).
• Indubitable: When I consider p
carefully, I am unable not to believe
it. Using my best, most careful
judgment, I judge that it is impossible
that it should be false: the proposition
‘is necessarily true each time it is
expressed by me, or conceived in my
• This is not a feeling of certainty, but a
use of rational insight.
The wax argument
• At first, our idea of the wax is of
something defined by its sensory
• But this is muddled: when I melt a
piece of wax, it loses all of its
original sensory qualities, yet I
believe it is the same wax.
• This shows our conception of
material objects, when clear and
distinct, is as changeable and
The external world: two
steps to go
• Meditation V: we can know that clear and
distinct ideas are true; so material objects
really are extended, if they exist at all.
• Meditation VI: We have experiences of an
external world, which must either be caused
by a real external world or God. God is not a
deceiver. Therefore material objects do
The Cartesian circle
• I am certain that God exists only
because I am certain of whatever I
clearly and distinctly perceive
• And yet…
• I am certain of whatever I clearly and
distinctly perceive only because I am
certain that God exists.
Descartes’ reply
• I can be certain of what I clearly and
distinctly perceive without knowing that God
exists, but only at the time that I perceive it.
– While I am clearly and distinctly perceiving some
particular proposition, then I am certain of that
proposition. But because of the possibility of the
evil demon, I lose this certainty as soon as I turn
my attention away from it, as I may be deceived
that I did perceive it clearly and distinctly.
Descartes’ reply
• God’s existence adds a general
certainty that what I clearly and
distinctly perceive is true.
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