1. Hume’s Fork A priori/ A posteriori Empiricism/ Rationalism
Knowledge Empiricism
Hume’s Fork
A priori/ A posteriori
Empiricism/ Rationalism

2. Hume’s Fork

3. Hume’s Fork
‘All the objects of human reason or enquiry fall naturally into two kinds, namely relations of ideas and matters of fact.’ (Hume, An Enquiry Concerning Human Understanding, Enquiry 1, section 4.)
Hume divides all objects of enquiry into two types – relations of ideas and matters of fact.
Relations of ideas are analytic truths. As such, they are necessarily true, and can be known a priori (deductively) and with certainty.
Matters of fact are synthetic truths. As such, they are only contingently true, and can only be known a posteriori (inductively), without certainty.

4. So… Hume’s empiricism about knowledge is that:
The only truths that we can know independently of experience are analytic truths (i.e. relations of ideas – trivial tautologies that tell us nothing about the world and what exists).
All substantive (synthetic) truths about the world, and what exists in it, must be known by experience.
So, all synthetic knowledge is a posteriori and all a priori knowledge is merely analytic.

5. Strengths with Empiricism
- It is supported by the plausible scientific idea that we need to use observation and experiment to find out about the world outside of us.
- It explains how we acquire some knowledge independently of experience, by saying that all such knowledge is about things which are internal to us (namely; ideas).
- It explains why there always seem to be problems with a priori arguments for substantive truths about the world (e.g. the ontological argument, the trademark argument, Plato’s arguments for the Forms, arguments for the existence of the soul, etc.)

6. Issues with Knowledge Empiricism
Descartes Sceptical Arguments:
As our synthetic knowledge is acquired solely a posteriori, through a process of induction and generalising from experiences (for example, the sun has risen in the past so the sun will rise in the future) it is not certain. Because this knowledge is not certain it can be challenged by scepticism.
While we may be certain that we are having sensory impressions we cannot be certain in moving from these impressions to beliefs about the world.

7. Issues with Knowledge Empiricism
Descartes highlights the uncertainty of information gained through the senses in the sceptical argument he uses within his Meditations.
Using his ‘three waves of doubt’ he attempts to doubt all of his beliefs in order to find a set of infallible beliefs from which to build a system of certain knowledge.

8. Descartes
‘Some years ago I was struck by how many false things I had believed, and by how doubtful was the structure of beliefs that I had based on them. I realised that if I wanted to establish anything in the sciences that was stable and likely to last, I needed – just once in my life – to demolish everything completely and start again from the foundations.’
Descartes, Meditations on First Philosophy, Meditation 1.

9. 3 Waves of Doubt
Descartes’ first wave of doubt (the argument from illusion) involves him doubting his senses because they have fooled him before. For example, he might believe he sees a friend walking on the other side of the road, only then to find out it wasn’t actually his friend. However, he realises that under normal conditions (for example up close and in normal lighting) he can trust his senses.

10. His second wave of doubt (the argument from dreaming) involves him doubting his senses because he could be dreaming. He argues that while he is experiencing sitting in front of a fire writing a book, he has often dreamed about this, so how can he tell apart dreaming and waking reality? However, he recognises that the materials of his dreams must be formed from reality. Therefore, there must be a real external world from which the ideas in his dreams can be built.

11. His third and final wave of doubt (the argument from the evil demon) involves him suggesting that there could be an evil demon deceiving him about everything. This demon might trick Descartes into believing there is an external world or of mathematical truths such as 2+2=4. (The matrix is a contemporary example of the evil demon problem – that we are deceived about the external world).

12. After using the three waves of doubt, Descartes finds that the only belief that he can be certain of is that he – as a thinking being exists – because even to doubt his existence involves thinking. Descartes concludes, ‘Cogito ergo sum’ – ‘I think therefore I am.’ It is from this belief that Descartes builds his system of knowledge.
‘So I shall suppose that some malicious, powerful, cunning demon has done all he can to deceive me... I shall think that the sky, the air, the earth, colours, shapes, sounds and all external things are merely dreams that the demon has contrived as traps for my judgement. I shall consider myself as having no hands or eyes or flesh, or blood or senses, but as having falsely believed that I had all these things.’ Descartes, Meditations on First Philosophy, Meditation 1.
Thus Descartes critiques the empiricist ideology of knowing about the world through the senses.

13. Additionally
If all knowledge of synthetic propositions can only be gained from sensory experience then it would follow that to know that God or morality existed you would have to have sensory experiences of God or morality and many argue that this is not possible.