2 Hume’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
3 Knowledge 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.
4 Causal 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.
5 A 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?
6 Descartes 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’.
7 The 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)=?
8 The 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.
9 Hume’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.