Thupten asked:

Can you explain how Hume justifies the existence of an external world?

Answer by Geoffrey Klempner

To readers of Hume’s Treatise of Human Nature this might seem like a rather strange question, given that Hume, in his philosophical system of ‘impressions and ideas’, seems to cast the very idea of an external world into doubt. We can’t even say we have the idea of an external world to sceptical about, given that the attempt to define an external object as one which ‘continues’ to exist when not perceived, and exists ‘distinct’ from perception, is flatly contradicted by the only evidence or basis for belief that we have, the nature of our subjective ‘impressions’ (‘On Scepticism With Regard to the Senses’, Book I, part 4, section ii).

Hume’s only solution was to leave such ‘cold, strained speculations’ behind and play a game of backgammon with his friends. At least, so goes the standard account.

But the standard account sits rather badly with Hume’s broad picture of the nature of his investigation. He is putting forward a ‘theory of human nature’, in the spirit of investigators of the time, like Newton with his ‘theory of motion’. Human nature can be studied by the philosopher, in a not dissimilar way to the study of the motions of the planets.

This is the very opposite of a ‘sceptical system of philosophy’. Hume saw his project as a contribution to human knowledge, opening our eyes to the truth about our place in the world, as governed by natural laws, not of motion, but of psychology.

And there is the clue: belief in an external world isn’t capable of rational justification. We just do believe in an external world, and every action we do proves this belief.

However, Hume wasn’t content to leave matters there. The most fascinating part of his philosophy is his ‘theory of fictions’, where he to some extent anticipates Kant in describing the way the human mind ‘constructs’ of external objects by means of the imagination, ideas that cannot be logically justified but which ‘work’ nonetheless. These ‘fictions’, when viewed from the standpoint of reason, have contradictory properties. As ideas, they clearly depend on us, and yet at the very same time they purport to represent ‘objects’ which exist independently.

Hume would say, what this shows is that philosophers have to once and for all learn their place. In place of reason and justification, there is naturalistic explanation. The external world doesn’t need to be ‘justified’.