© michael lacewing descartes on certainty (and doubt) michael lacewing...
TRANSCRIPT
Hyperbolic doubt
• To find the truth, need to avoid believing error - so only believe what is ‘indubitable and certain’
• What is indubitable appears to be what we are unable to doubt
What is certainty?
• Subjective, psychological feeling of conviction
• 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 sleep’.
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 mistakes.
• 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).
Certainty
• 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 mind’.
• 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 properties.
• 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 extended.
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 exist.
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.