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
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.
Analytic/synthetic A proposition is analytic if it is true or false just in virtue of the meanings of the words. A proposition is synthetic if it is not analytic, i.e. it is true or false not just in virtue of the meanings of the words, but in virtue of the way the world is.
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.
Hume’s challenge Why and how do we move from ‘this is what happens here and now’ to ‘this is what happens generally’? Similar causes have similar effects, and so the future will be like the past and events elsewhere that we haven’t actually experienced will be like events we have experienced. How do we know this?
Past and future Because in the past, similar causes had similar effects. But how do we know similar causes will have similar effects in the future? Past experience can give me ‘direct and certain information of those precise objects only, and that precise period of time, which fall under its cognizance’.
Hume’s solution We draw the inference from cause to effect without reasoning or argument, but on the basis of a principle of the ‘imagination’ - custom - that has bound the two ideas – of the cause and of the effect – together in our minds. Custom is a natural instinct of the mind, a disposition we simply have in the face of experience of constant conjunction.
Expectation Potassium in water (usual) - Potassium in water (unusual) When we experience something that has been a cause in the past (one billiard ball striking another), we immediately believe that its usual effect is about to occur.
Scepticism Without custom, we would be unable to draw causal inferences. Without causal inference, we have no knowledge of anything beyond what was present to our senses and memory. But custom is not reason. So we have no reason to believe that what we haven’t experienced will be like what we have.
External world 1.We are naturally disposed to believe in the external world, and at first we think that our impressions perfectly resemble it. 2.On reflection, we don’t suppose a table gets smaller as we move away. 3.So we must accept that what is immediately available to the mind is only ideas, which don’t resemble objects perfectly; yet we continue to think that the objects represented persist independently of our impressions.
External world 4.But now we must wonder how we can show that our impressions must be caused by such independent objects! 5.Experience can’t show this, because all that experience has available is the impressions themselves, not the connexion between impressions and objects. 6.The belief in the external world, therefore, is groundless.
Was Hume a sceptic? Hume clearly claimed we do have knowledge as a result of causal inference, and since knowledge of the external world is a prerequisite for this, we have knowledge of that, too. Hume has two models of ‘knowledge’: –Practical: knowledge rests on custom and other instincts, such as belief in the external world. –Reason only (deduction): Knowledge only of what is immediately present to our senses, and what we can remember.