2Hume’s fork We can only have knowledge of Relations of ideas Matters of factRelations of ideas are a priori and analyticMatters of fact are a posteriori and synthetic
3Knowledge of matters of fact Knowledge of matters of fact is always a posteriori and synthetic.We gain it by using observation and employing induction and reasoning about probability.The foundation of this knowledge is what we experience here and now, or can remember.
4Causal inferenceAll our knowledge that goes beyond what is present to our senses or memory rests on causal inference.We can’t know what causes what without investigating experience.
5A priori knowledge of what exists Some rationalists, e.g. Plato and Descartes, challenge Hume’s claim; we can use reason to prove what existsA priori demonstration and intuitionDemonstration = deductionIntuition - e.g. how do you follow an argument?
6Descartes on the physical world We can be deceived by our senses.There are ‘no certain indications by which we may clearly distinguish wakefulness from sleep’.
7The cogitoI am certain that I think, I exist. I am a thinking thing. But I am not certain I have a body.So I can exist without a body. (A priori reasoning)=?
8The physical world What causes our experiences of the physical world? Physical objectsMeEvil demonGodNot me: I would know if I imagined themNot evil demon or GodThese options would entail that God is a deceiverGod exists and is not a deceiverTherefore, physical objects exist.
9Hume’s response‘Nothing is demonstrable, unless the contrary is a contradiction.’ But it is not a contradiction to deny that something exists, e.g. that God does not exist.So a priori reasoning can’t tell us what exists.Objection: this assumes that a priori reasoning can only establish analytic truths. There could be synthetic a priori truths, and ‘God exists’ could be one of them.