Rene Descartes

Rationalists claim that synthetic a priori exists and have made attempts to prove this. They use deduction a lot and call it 'demonstration'.

The Cogito - attempts to prove he exists in an a priori way. 

P1 I can't doubt that I can doubt

P2 I can doubt

P3 To doubt, I must exist

C Therefore I exist

Criticisms..

• He is already assuming he exists by using the word "I"
• Why assume it is us that doubts?
• Only proves that while we're doubting, we exist

The Ontological Argument - attempts to prove that God exists in an a priori way.

P1 I have the idea of God

P2 God is a being with all perfections 

P3 Existence is a perfection

C Therefore God exists

Criticisms..

• He is already assuming that he exists by using the word "is

          Descartes: Might defend himself by saying that he is referring to only the idea of God

• God may not be perfect, this is too culturally specific (religious)

• Kant: Existence is not a predicate/word that gives detail. E.g. "I have a coat, it's red" -- Red would be a predicate. "I have a coat, it exists". Exists would not be a predicate.

• Gaunilo: This argument proves that anything perfect can exist. The perfect baby exists because it has all perfections and because existence is perfection, it exists. 

Deduction criticisms..

• A way of making people accept his argument because we have a 'rational intuition' to accept deductive arguments. 

       However, this doesn't make all rationalists' arguments fail.

The Wax Argument - attempts to prove that we can only gain true knowledge of wax from our rational intuition.

Experiencing wax gives us the shape, texture, colour, size, smell and so on. However, when wax is melted, its characteristics change. Only our rational intuition or "faculty of judgment" helps us understand this is the same wax. Therefore, experience isn't the source of all knowledge. 

Criticisms..

• When we experience the wax being melted, we know it is the same wax. Descartes seems to skip past the idea of the wax being melted and focuses on the end result more. 
• Using a spectrometer will show both solid and melted wax as having the same molecule structures. From experiencing these, we will gain the idea that they are the same.