1. Empiricism on a priori knowledge
Michael Lacewing
© Michael Lacewing

2. A priori knowledge
A priori: knowledge that does not require (sense) experience to be known to be true (v. a posteriori)
It is not a claim that no experience was necessary to arrive at the claim, but that none is needed to prove it.

3. Analytic and synthetic propositions
An analytic proposition is true or false in virtue of the meanings of the words.
Not all analytic propositions are obvious: In five days’ time, it will have been a week since the day which was tomorrow three days ago.
A synthetic proposition is one that is not analytic, i.e. it is true not in virtue of the meanings of the words, but in virtue of the way the world is.

4. Rationalism v. empiricism
Rationalism: we can have a priori knowledge of synthetic propositions about how things stand outside the mind.
Empiricism: we cannot.
Empiricists do not claim there is no a priori knowledge, because they grant that knowledge of analytic truths is a priori.

5. Empiricism on a priori knowledge
For any field of enquiry, either
Knowledge is possible, but empirical, not a priori
Knowledge is possible and a priori, but analytic
Knowledge is not possible

6. Moral knowledge
Which category do moral judgments, e.g. ‘murder is wrong’, ‘generosity is good’, fall into?
Analytic truths? No
Through (sense) experience? Maybe not
So synthetic a priori?

7. Hume and Ayer
Hume: we can only know ‘relations of ideas’ (analytic and a priori) and ‘matters of fact’ (synthetic and a posteriori)
Ayer’s verification principle: all meaningful statements are either analytic or empirically verifiable

8. Hume and Ayer on ethics
Hume: ‘when you pronounce any action or character to be vicious, you mean nothing, but that… you have a feeling… of blame from the contemplation of it. Vice and virtue… are not qualities in objects, but perceptions in the mind’
Ayer: ‘If I say to someone, “You acted wrongly in stealing that money”…I am simply evincing my moral disapproval of it. It is as if I had said, “You stole that money,” in a peculiar tone of horror.’
Moral judgments are neither true nor false, but expressions of our feelings and what we care about

9. Mill on ethics
Mill: an action is right if it creates greater (or equal) happiness than any other action in that situation
So whether something is right or not is an empirical fact
But how do I know that happiness is good?
What is good is what is desirable. Our evidence about what is desirable must come from what we desire.

10. Mathematics Mathematics can’t be empirical
2 + 2 = 4, while obviously learned through experience, is not justified by experience – it is necessarily true
Since mathematics is a priori, most empiricists argue it is analytic - we don’t allow true mathematical claims to be false, they are true by definition
Objection: this doesn’t do justice to mathematical discoveries

11. Metaphysics
Kant: a priori synthetic truths are about the way experience must be for us, e.g. ‘Nothing can be coloured in different ways at the same time in the same part’
If this is analytic, it is made true by rules of language. But is this the (arbitrary) source of how we experience things?

12. Rational intuition?
How could we gain knowledge of ‘metaphysical’ truths? ‘Reason’
But how does ‘reason’ work here? What is rational ‘intuition’ into how things are? Is it reliable?