1. Descartes Meditations

2. Knowledge needs a foundation Descartes knows he has false beliefs, but he does not know which ones are false So, we need a method to sort out the false beliefs from the true ones

3. The Method of Doubt Treat all beliefs that are not absolutely certain as if they were false. Try to determine if there is anything that can be known with absolute certainty, any belief that is immune to doubt.

4. You don’t need to go through all your beliefs one by one to see if they are doubtful. You can show a belief is doubtful if you can show that its source is unreliable What is the source of most of our beliefs?

5. Doubting the senses Sense experience is the source of most of our beliefs So if we can show that sense experience is doubtful, then we can show that all the beliefs based on sense experience are doubtful.

6. The Dream Argument When we dream we have a sense experience. When we dream we have a sense experience. The objects of dreamed sense experience are unreal (dreams are a deceptive sense experience) If the senses deceive us in dreams, they can deceive us any time If the senses deceive us in dreams, they can deceive us any time Therefore, all the beliefs based on sense perception are in doubt.

7. A priori and A posteriori Knowledge A posteriori knowledge is knowledge based on the senses. A priori knowledge is knowledge that is not justified on the basis of sense experience If a belief is justified no matter what sort of sense experience you have, then it is a priori.

8. Possible Examples of A priori Knowledge There cannot be a round square 2+2=4 The sum of the angles of a triangle is 180 degrees All bachelors are unmarried. Nothing can be blue all over and red all over at the same time in the same respect.

9. Descartes: mathematical and geometrical truths can be known even if I am living a huge consistent dream/hallucination. Therefore these are a priori and not called into doubt by the dream argument

10. The Evil Demon! It is possible that there is a very powerful deceiver, a being that aims to deceive me about everything I believe. Therefore it is possible that everything I believe is false. This argument applies to a priori and a posteriori beliefs. The demon can mess with how you think, as well as what you perceive.