David Hume (1711-1776)

Berkeley thought that material objects were just collections of ideas partly because he used the word 'ideas' to cover everything that the mind perceives, from abstract ideas about God or physics to sensations of color, sound, smell, etc.  Hume thought that we should discriminate a little more than that.  So he distinguished between impressions, which are what we actually sense or feel while we are having a sensation or feeling, and ideas, which are slightly faded copies or memories of past impressions and also made up combinations of ideas and impressions (such as the made up idea of a golden mountain).

All ideas come from impressions, Hume maintains.  Any alleged idea that cannot be analyzed into its original component impressions is bogus and has no real meaning.  We should remove it from our thinking and thereby remove all needless obscurity from philosophy.

One such idea, Hume argues, is Descartes's idea of the self (or mind or soul).  Looking outside ourselves we do not, of course, smell, see, etc. any such thing as a non-physical object.  Looking inside all we find are ideas and impressions (i.e. ideas, desires, memories, emotions, etc.).  We find no mysterious substance that contains all these ideas and impressions.  So far from being the thing of whose existence we are the most certain (apart from God) as Descartes thinks, this 'self' that he refers to does not exist at all.  The idea is meaningless.  It corresponds to nothing in our experience, or even in our imagination.  The only self we have is a bundle of changing impressions and ideas.

Like ideas, impressions are in the mind.  We take (some of) them to be caused by, and to represent accurately, physical objects outside the mind.   But is this belief in an external world corresponding to our sensory impressions rationally justified?  Hume says it is not.  All we know is our impressions and the ideas we derive from them.  We can nothing of the world outside the mind, or even whether there is such a world.  But that is all right.  Hume does not care.   We do believe in the external world, even though this is not rational.  This shows not that our belief is wrong but that we are not as rational as some philosophers, especially religious ones who believe we are made in God's image, have liked to pretend.

Another example of our non-rationality is our tendency to reason inductively.  Induction is reasoning from some limited set of examples to a general conclusion about all similar examples.  A bad example is prejudice based on one or two bad experiences.  A good example is our belief that the sun will rise tomorrow, based on countless days' experience. 

According to Hume, it is not rational to believe in something without logical proof.  My impressions are my impressions, so I cannot logically deny their existence, even though I can doubt whether they are hallucinations, reliable perceptions or whatever.   But my impressions are only of the present, and my ideas all come from the past.  What can they possibly tell me about the future?  Nothing at all, unless we assume, quite non-rationally, that the future will be more or less the same as the past (e.g. that the laws of physics will not change drastically, etc.).   But logic does not justify any such assumption.  We do not even know that the sun will probably rise tomorrow, from a strictly rational point of view.   Hume's point is not that the sun might not rise tomorrow.  It is that our perfectly reasonable, sane, common sense belief that it will rise is not also a rational belief in the strict sense.   

Hume is similarly skeptical about the rationality of our belief in causation.  The idea of a cause, he says, is not just the idea of "constant conjunction, " i.e. of one kind of event (e.g. blows to the head) always being followed by another kind of event (e.g. pain being experienced).  It is the idea that the first event makes the second event inevitable or necessary.  But what do we actually see with our eyes?  Constant conjunction, not necessary connection.  Where do we get this idea?  Logic does not tell us that constant conjunctions must be necessary.  They could be coincidences, or the real cause might be God, not the blow to the head or whatever else seems to cause the event in question.   Whenever we see events of type A constantly followed by events of type B, the human mind (what we might call common sense) is such that we cannot help thinking of B whenever we think of A.  This tells us something about the nature of our minds, though, not the nature of the world outside the mind.