A posteriori Knowledge A priori knowledge A posteriori knowledge is based on experience. A posteriori knowledge is based on experience. A priori knowledge is knowledge that is not based on experience. A priori knowledge is knowledge that is not based on experience. If you can know something just by thinking about it, without consulting how the world appears to you, that is a priori knowledge. If you can know something just by thinking about it, without consulting how the world appears to you, that is a priori knowledge.

Possible examples of a priori knowledge “bachelors are unmarried” “bachelors are unmarried” 2+2=4 2+2=4 There cannot be a round square There cannot be a round square Any logical truth—the connection between premises and conclusion in a valid argument Any logical truth—the connection between premises and conclusion in a valid argument

The dream argument calls into question knowledge based on experience, but it leaves a priori knowledge untouched The dream argument calls into question knowledge based on experience, but it leaves a priori knowledge untouched But the evil demon argument questions also a priori knowledge But the evil demon argument questions also a priori knowledge

The EVIL DEMON! It is possible that there is a demon, with nearly godlike powers, who aims to deceive me. It is possible that there is a demon, with nearly godlike powers, who aims to deceive me. Therefore it is possible that all my beliefs, are the result of the deceptive powers of a demon Therefore it is possible that all my beliefs, are the result of the deceptive powers of a demon Therefore I cannot trust any of my beliefs Therefore I cannot trust any of my beliefs

Meditation 2 Descartes now tries to discover some belief or beliefs that can be known with certainty. These beliefs would have to be true even if there is a demon Descartes now tries to discover some belief or beliefs that can be known with certainty. These beliefs would have to be true even if there is a demon He finds one: I exist. He finds one: I exist. But could an evil demon deceive him (or you) about your own existence? But could an evil demon deceive him (or you) about your own existence?

Descartes answers no. Deception requires that there be something to be deceived. Even if all my thoughts are mistaken, the thoughts must exist in order for them to be mistaken Everytime I reflect on my thinking I can know with absolute certainty that I exist.

But what sort of thing is this “I” Am I a rational animal? Am I a rational animal? Am I a human being? Am I a human being? What is essential to this “I” that I know to exist? What is essential to this “I” that I know to exist? Essentially, what I am is not human being, not rational animal, but a thinking thing Essentially, what I am is not human being, not rational animal, but a thinking thingWhy?

The knowledge that I am human or an animal, or even have any body, is still in doubt. A demon can deceive me about these things. So I cannot know anything about my physical being. But there is something I know. What I know is the residuum, it is what is left over after all that can be doubted is doubted—thinking, consciousness. The knowledge that I am human or an animal, or even have any body, is still in doubt. A demon can deceive me about these things. So I cannot know anything about my physical being. But there is something I know. What I know is the residuum, it is what is left over after all that can be doubted is doubted—thinking, consciousness.

Solipsism! Solipsism is a view that can only be stated in the first person. Solipsism is a view that can only be stated in the first person. My mind is all that exists, everything else only exists as I experience it. My mind is all that exists, everything else only exists as I experience it. There are few if any solipsists. But is there any way to refute solipsism?

The piece of wax Therefore we know the mind with much more clarity than we know the body. Therefore we know the mind with much more clarity than we know the body. But this is hard to believe. Its easy to think that knowledge of the physical world is much more concrete, “tangible,” and that the mind is somehow mysterious But this is hard to believe. Its easy to think that knowledge of the physical world is much more concrete, “tangible,” and that the mind is somehow mysterious The piece of wax example shows us this is an error The piece of wax example shows us this is an error

Take a piece of wax, melt it. Take a piece of wax, melt it. All the sensible properties change All the sensible properties change Yet we still believe, we still take the wax to be the same thing Yet we still believe, we still take the wax to be the same thing This shows: [supposed] knowledge of physical things is based on the mind. We need to think to know the physical world This shows: [supposed] knowledge of physical things is based on the mind. We need to think to know the physical world Our concept of the physical world is not a bundle of sensible qualities. Our concept of the physical world is not a bundle of sensible qualities.

What we understand when we understand that the piece of wax is the same is a “flexible, extended substance” What we understand when we understand that the piece of wax is the same is a “flexible, extended substance” It is what underlies or causes our perceptions, not what we directly perceive It is what underlies or causes our perceptions, not what we directly perceive Descartes is foreshadowing future conclusions in the Meditations. Descartes is foreshadowing future conclusions in the Meditations.