Humes fork We can only have knowledge of –Relations of ideas –Matters of fact Relations of ideas are a priori and analytic Matters of fact are a posteriori and synthetic
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.
Causal inference All our knowledge that goes beyond what is present to our senses or memory rests on causal inference. We cant know what causes what without investigating experience.
A priori knowledge of what exists Some rationalists, e.g. Plato and Descartes, challenge Humes claim; we can use reason to prove what exists A priori demonstration and intuition –Demonstration = deduction –Intuition - e.g. how do you follow an argument?
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.
The cogito I 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) = ?
The physical world What causes our experiences of the physical world? –Physical objects –Me –Evil demon –God Not me: I would know if I imagined them Not evil demon or God –These options would entail that God is a deceiver –God exists and is not a deceiver Therefore, physical objects exist.
Humes 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 cant 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.