REASON AND EXPERIENCE

Intro: Like John Locke before him, David Hume believed that our knowledge derives primarily from experience. However, he also argued that we can never know anything about the world with certainty

Natural assumptions

David Hume (1711–1776) was primarily interested in epistemology (the nature of knowledge), rather than metaphysics (the nature of the universe). In An Enquiry Concerning Human Understanding, he set out to examine the way that human psychology determines what we can and cannot know, and in particular what we can and cannot know for certain.

Although an empiricist – that is, he believed that experience is our primary source of knowledge – Hume conceded that many propositions, such as mathematical axioms, can be arrived at by reason alone and cannot be doubted: to doubt that 2+2=4 is to fail to understand its meaning. However, he argued that such truths tell us nothing about the world: they simply express relationships between ideas. To gain knowledge about the world we need experience, but Hume argues that such knowledge can never be certain. We are therefore caught on the prongs of a fork: on the one hand, we have certainty about things that tell us nothing about the world; on the other hand, our knowledge about the world is never certain.

Hume argues that it is human nature to make assumptions about the world, especially that it is predictable and uniform. We assume, for example, that when a brick is thrown at a window the brick “causes” the window to smash. However, Hume argues that all we know for certain is that throwing a brick at a window is regularly followed by the window smashing. We never perceive causes, he says, but only a “constant conjunction” of events – that is, the regular occurrence of certain events following others. We only imagine a “link” between them.

Hume is not saying we are wrong to make assumptions – life would be impossible without them. Rather, he is suggesting that we should recognise the extent to which assumptions govern our lives, and not confuse them with the truth.

Relations of ideas

Statements of this kind are necessary truths, which means that they cannot be contradicted logically. For example, it is not possible to say that the angles of a triangle do not add up to 180 degrees, or that 2 plus 2 does not equal 4. We can be certain of such truths, but they tell us nothing about the world: they merely express relationships between ideas.

Hume’s fork

For Hume, there are two kinds of truth: “relations of ideas” and “matters of fact”. The former are true by definition, while the latter depend on the facts. Philosophers call this distinction “Hume’s fork”.

Matters of fact

Statements of this kind are contingent, which means that their truth or falsehood depend on whether or not they represent the facts. For example, it is not illogical to deny the statements “It is snowing” or “I have a cat”. Their truth depends simply on the current state of the weather and whether I own a cat or not.

“Custom, then, is the great guide of human life.” – David Hume, An Enquiry Concerning Human Understanding (1748)

THE PROBLEM OF INDUCTION

Hume argued that general statements such as “The Sun rises in the east” are logically unjustified because we cannot prove that the Sun will not rise in the west tomorrow. This also means that scientific claims, such as “The Moon orbits the Earth”, are unjustified because we may discover, for example, that the Moon may also behave in a different way tomorrow. Such statements are known as “inductions”, because they use the inductive method of reasoning – that is, they make general claims based on a limited number of particular cases or events. There are different types of inductive argument which will be examined in detail in a future entry.

NEED TO KNOW

. According to Hume, the difference between mathematics and the natural sciences is that mathematical truths are what he calls “relations or ideas”, or necessary truths, whereas scientific truths are contingent, or conditional, “matters of fact”.

. Half a century before Hume, Gottfried Leibniz made a similar distinction between truths of reasoning and truths of fact. Leibniz was an accomplished mathematician. He invented calculus (which Isaac Newton was also instrumental in) and was characterised as an optimistic philosopher. He believed that God is supremely perfect, and that ours is the best possible world – one in which the modern “monads” exist in harmony. The word “monad” is derived from the Greek word monas, meaning “unit”, which Leibniz borrowed to describe the fundamental units of existence. He distinguished “truths of reasoning” from “truths of fact”.

. Immanuel Kant and later philosophers distinguished between analytic statements, whose truth can be established by reasoning alone, and synthetic statements, which are verified by reference to the facts.