Descartes on Certainty (and Doubt) Michael Lacewing [email protected] © Michael Lacewing 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.