Dave Hume was born in Edinburgh in 1711 and was a central figure of the Scottish Enlightenment.

The philosophical context he was born into was one that was trying to understand the world scientifically. Francis Bacon had argued for a science based on experience – an experimental inductive method of observing and recording. Galileo had disproved the geocentric model of the universe – that everything revolved around the earth. And Newton had laid down the laws of motion and classical mechanics in physics.

Hume wanted to do the same for the human mind.

He worked for various merchants and governments but spent most of his life writing, finding success in his day for The History of England.

He published A Treatise of Human Nature in 1739 when he was just 28. It became one of the most influential works in philosophy but was largely ignored in his day.

He later distilled and clarified his thought into the shorter An Enquiry Concerning Human Understanding, which contains most of its key themes.

He was friends with Adam Smith and Rousseau, whom he had a famous falling out with after the latter came and stayed with him.

Locke and Berkley had contributed to building a picture of human reason based on the scientific method and empiricism. Bertrand Russell describes Hume as developing it to its ‘logical conclusion’.

Hume’s influence can be divided into two parts: his theory of mind, human understanding, or psychology; and the moral philosophy that follows from it.

It is necessary to understand the former to make sense of the latter, and the relatively short Enquiry is a clear, witty, introduction to his thought.

After an obligatory philosophical introduction that criticises how philosophers have approached their subject matter in the past, Hume ends the first section with a call to ‘undermine the foundations of an abstruse philosophy, which seems to have hitherto served only as a shelter to superstition, and a cover to absurdity and error!’

He begins by dividing the faculties of the mind into two: impressions and ideas.

Impressions are like inputs – from the senses, from hunger, from emotion – the foundational feelings of the mind – and ideas are recollections of these impressions, which are inevitably weaker.

We receive the impression of a tree. We recollect the idea of a tree.

Ideas can combine impressions – we might see gold and see a mountain and combine them into a golden mountain. This is how our imagination works.

All ideas, though, however complicated, can be traced back to original impressions. This is what makes him an empiricist – all knowledge comes through the senses.

Even the idea of God, for example, is the impression of wisdom, intelligence, power et cetera, all combined and multiplied.

In the next section Hume asks how we create ideas out of impressions. All ideas, again, however complicated, must have been made through some logical connection – and for Hume there are only three: resemblance, contiguity, cause and effect.

For resemblance, we might put the impressions of two different trees together into one category of tree.

For contiguity, we might see a bird perched on the tree and make a connection.

For cause and effect, we see a pan over fire boils water and hold this idea.

All ideas are formed only by these associations.

Human reason, what the mind can create, can only be the product of either matters of fact – impressions through the senses – or relations of ideas – combining those impressions together logically through resemblance, contiguity, and cause and effect.

This is what became known posthumously as Hume’s fork.

That knowledge can only be acquired this way leads Hume to his scepticism.

Everything we know is based on experience and cause and effect.

We see the sun rise, it gets light. We eat food, we stop getting hungry. We put the water over fire, it heats.

But because its only based on experience there is never any real, rigorous, philosophic guarantee that it will happen again in the future.

We only ever see A leading to B – we never really see the cause of something.

We see a billiard ball hitting another and the other moving, but event A only leads to event B. You cant see a necessarily link, only correlation.

The effect is not a necessary part of the cause. There is nothing inherently necessary in the billiard ball that equals the cause and the effect of the event.

Our mind can only operate on the probability that an event will repeat in the same way. We are confident that the sun will rise tomorrow because we have experienced it so often. There is no way of being certain, though.

This, still debated to this day and troubling for scientists, is the problem of induction.

Our minds act as though we can predict the future – but they’re only acting, even the most scientific minds, on past experience.

Science doesn’t guarantee anything – it only ever shows the probability of events recurring.

This – what he refers to as modest scepticism – gives us the limits of human reason.

But it also leads Hume to another question: how do we live our lives effectively with no guarantees of what we know from experience repeating next time?

He says that every time we see cause and effect repeat it grows into custom or habit, which he calls ‘the great guide of human life’.

Hume said, ‘All belief of matter of fact or real existence is derived merely from some object, present to the memory or senses, and a customary conjunction between that and some other object’.

We put pencil to paper, it leaves a mark. We repeat, it does it again. This leads to a custom.

We peal a banana, the inside is softer. We do it again. And form a habit of eating.

We can create new customs and habits by mixing old experiences together – using the associations of resemblance, contiguity, cause and effect.

This pen resembles the pencil – maybe it does the same thing.

There is a tree that resembled the first tree – maybe a bird will be sat in it again, as trees and birds are contiguous.

He says that while there is no such thing as chance – chance being only the ignorance of the human senses in seeing the real cause and effect – we nevertheless have to organise our habits around a judgement of probability

We experience cold when the sun’s at its lowest, we experience that when it’s farther from the earth, snow is more likely, or when certain wind conditions blow storms over us we can intuit the likeliness of snow tomorrow and through habit or custom take a thick coat out with us.

But we can never see necessary connection. We can never know for sure that it will snow tomorrow. And, logically, we can never know that the sun will rise.

We can only expect.

Hume writes, ‘We say, for instance, that the vibration of this string is the cause of this particular sound. But what do we mean by that affirmation? We either mean, that this vibration is followed by this sound, and that all similar vibrations have been followed by similar sounds: Or, that this vibration is followed by this sound, and that upon the appearance of one, the mind anticipates the senses, and forms immediately an idea of the other’.

This view is troubling for many – of course we know the sun will rise tomorrow! – but it’s also logically airtight, and philosophers attempting to dispute it is what made Hume’s work so influential after his death.

The text finishes with a commentary on religion and miracles, views he was ostracised for in his day, and which also serves as an analogy for a critique of any fantastical views.

A wise man, he says, ‘proportions his belief to the evidence’, and ‘proceeds with more caution: He weighs the opposite experiments: He considers which side is supported by the greater number of experiments: To that side he inclines, with doubt and hesitation; and when at last he fixes his judgment, the evidence exceeds not what we properly call probability. All probability, then, supposes an opposition of experiments and observations, where the one side is found to overbalance the other, and to produce a degree of evidence, proportioned to the superiority’.

Contrary testimony, witnesses contradict each other, or can have doubtful character.

If a miracle is the suspension of the laws of nature, what is more likely?

‘When any one tells me, that he saw a dead man restored to life, I immediately consider with myself, whether it be more probable, that this person should either deceive or be deceived, or that the fact, which he relates, should really have happened. I weigh the one miracle against the other; and according to the superiority, which I discover, I pronounce my decision, and always reject the greater miracle’.

Hume finishes with his famous lines: ‘If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: For it can contain nothing but sophistry and illusion’.

The Enquiry can be thought of as an introduction to the Treatise, which contains, in three volumes, a more detailed exploration of understanding, passions, and morals, respectively.

Sources

David Hume, An Enquiry Concerning Human Understanding

A.J. Ayer, Hume: A Very Short Introduction

Anthony Kenny, The Rise of Modern Philosophy

Bertrand Russell, History of Western Philosophy