David Hume developed the empirical philosophy of Locke and Berkeley to its logical conclusion, arriving at a degree of scepticism which makes his ideas seem absurd to most. Mocking Hume is a favourite pastime of some rationalists according to Russell, but this does not mean that Hume is wrong, or that his ideas are not important.
Firstly some thoughts on one of the key questions within empiricism by way of introduction, although it does not directly relate to Hume’s ideas. Empiricism says that all knowledge starts from what we can observe. Russell explores the arguments as to whether we can infer additional knowledge from what we have observed. We could take the position that inferences from observed knowledge are impossible. It is impossible to deduce with certainty the existence of one object or event from the existence of another, because all objects are capable of existing independently of one another. In this case the known world is limited to what we have directly observed. Another position is to say that we can infer things analogous to our own experiences, such as things experienced by other people but not by ourselves, that are described to us. Another position, which Russell considers to be common sense, is to say that there are objects and events that exist and that no one observes. From a scientific perspective, unobserved objects and events can be considered to have existed from a probabilistic basis or based on causal laws. An example would be the big bang, which obviously cannot be observed, but the causal laws which demonstrate its existence are based on scientific observations.
Ideas & Impressions
Hume divided perceptions (observations) into two categories: ideas and impressions. The act of observing something leaves an impression on us, from which we can derive ideas. This is sometimes a physical impression, such as the light that travels into our eye when we see something. Simple ideas are identical to their corresponding impression, but we can form more complex ideas by combining multiple impressions. To give Russell’s example, we can imagine a winged horse without observing one, but the idea is made up of several impressions (i.e. of horses and wings) which are from observations. Hume also believed that we cannot get an impression of the individual self, and therefore cannot have knowledge of the self. We can only know ourselves as a collated bundle of perceptions, which are constantly in motion. (This would appear to undermine modern Liberalism, which says that ultimate authority comes from the inner self of every individual; how can we trust our inner self if we do not understand it?). To express this in more tangible terms, no one can observe their own brain and its workings directly, and therefore form a single simple idea of it. We can form a complex idea of our brain made up of multiple impressions (e.g. inferences from the study of other human brains). This, if true, does not prove there is no soul, but it does mean that we cannot know anything about the soul.
Hume has a theory on uncertain knowledge, which includes everything except direct observation, logic & mathematics. Knowledge of cause and effect is placed within the realm of uncertain knowledge, which is contrary to Descartes and his followers who said that cause and effect are logical necessities, and amounts to an attack on determinism. The reason one thing causes another is not discernible from the understanding of each of the two things, so cannot be deduced. The only way to have knowledge of cause and effect is to observe one thing causing another. According to Hume, we cannot infer that an observed cause and effect will apply to similar situations, including future situations. The expectation of a future event is merely a belief based on past experience, and not a certainty. This feels reasonable, but Hume goes further by suggesting that the frequent association of two objects or events in the past gives no reason to think that they are likely to be associated in the future. This means that no belief regarding a future event can be based on reason. The two positions can be illustrated with an example. If I eat an apple, I expect to experience a certain taste. It is reasonable to say I cannot be certain of how it will taste based on previous experience, but it is also highly likely to taste similar to previous apples that I have eaten, to the extent that for all practical purposes this can be treated as a certainty. For Hume to suggest that we should have no expectation as to what an apple will taste like and to dismiss all previous experience is absurd, but difficult to definitively refute. Empiricists have generally accepted the principle of induction – that knowledge derived from observation that is only probably true can be considered valid, even though it is not certain. The principle of induction itself cannot be known through observation, so is accepted on rationalist rather than empirical grounds. This is why a consistent approach to empiricism (as held by Hume but not by Locke) rejects induction, leading to a desperately sceptical situation where almost nothing can be known except mathematics and what can be directly observed.
Subsequent philosophers either rejected Hume’s scepticism without really refuting it, or they have accepted that no belief is based on reason but said that feeling is superior to reason as a basis for knowledge. The latter group would become known as the Romantics, and they would hold convictions that were quite different to what had gone before.