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. Hume divides the areas of human understanding into two camps.
Hume’s fork
Hume divides the areas of human understanding into two camps.
“All the objects of human reasoning or enquiry fall naturally into two kinds, namely relations of ideas and matters of fact.”
work on your own to draw this table and sort which types of truths below are ‘Relations of Ideas’ and which are ‘Matters of Fact’.
Analytic truths
Synthetic truths
A priori truths
A posteriori truths
Necessary truths
Contingent truths
All bachelors are unmarried men
The sun will rise tomorrow
Pythagoras’ theorem
All bachelors are rich men.
Relations of ideas
Matters of fact

6. Hume’s fork Relations of ideas Matters of fact Analytic Necessary
A priori
Certain
Examples: Pythagoras’s theorem; All Bachelors are unmarried men.
Synthetic
Contingent
A posteriori
Probable
Examples: the sun will rise tomorrow; all bachelors are rich men.

7. Hume’s empiricism about knowledge
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 truths about the world, and what exists in it, must be known by experience.

9. Sheet to look at sheet

10. Weaknesses of Hume’s empiricism
The main problem is that we may not be able to gain true knowledge of the world through our senses.
Rationalist philosophers argue that our senses are not trustworthy enough to give us reliable knowledge of the world.
The rationalist philosopher Descartes argued for this using his three waves of doubt.

11. Other problems for Hume’s empiricism
It implies that mathematical truths are analytic; but this is questionable: do the ideas of ‘2’ ‘+’ and ‘2’ contain the idea of ‘4’?
It has some trouble with moral truths too, since these seem neither analytic (the idea of ‘murder’ doesn’t contain the idea of ‘wrong’) nor discovered by observation and experiment (we can’t see that something is good/bad, right/wrong).
It may be self-defeating, since the view itself seems neither analytic, nor grounded in sense-experience.

12. 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.)

13. 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.

14. 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.

15. The Crux of The Issue… What is the difference between knowledge empiricism and knowledge rationalism?
Empiricists claim that all knowledge of synthetic propositions is a posteriori, while all a priori knowledge is of analytic propostions. So, anything we know that is not true by definition or logic alone, we must learn and test through the senses.
Rationalists deny this, claiming that there is some a priori knowledge of synthetic propositions, either because this knowledge is innate or because we can gain such knowledge using reason rather than sense experience.
Many rationalists add that the synthetic a priori knowledge we gain through reason or innately cannot be arrived at in any other way.
They may also argue that it is superior, for example by being more certain, to the knowledge or beliefs we gain through the senses.
Empiricists deny and rationalists assert that we can gain a priori knowledge of synthetic propositions that are about things other than one’s own mind.

16. 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.

17. Descartes – 3 waves of doubt
Old System of Beliefs
Need to demolish it because…

18. 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.

19. 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.

20. 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).

21. 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.

22. 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.