1. How do we know what exists?
Michael Lacewing
© Michael Lacewing

2. Hume’s 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

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 inference
All 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 exists
A priori demonstration and intuition
Demonstration = deduction
Intuition - 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 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) = ?

8. 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.

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.