David Hume is a Scottish philosopher from the 18th century. He sought to steer philosophy away from its speculative tendencies and refocus it on the scientific method.
Hume is an empiricist, holding that knowledge ought to be sought through observation of the physical world rather than abstract theorizing.
Observation entails both impressions and ideas — Hume’s two categories of perception, or “mental content.” Impressions impart sensory data and elicit emotions while ideas interpret impressions using thought and reason. This notion that ideas are derived from impressions is known as Hume’s Copy Principle, which Hume describes in the following way:
“All our simple ideas in their first appearance are derived from simple impressions, which are correspondent to them, and which they exactly represent.”
To support his claim, Hume challenges us to imagine an idea without its corresponding impression. For instance, is it possible to know that the sun emits heat without sensing the warmth given off by its rays?
Hume argues that a failure to recognize the Copy Principle has led philosophers to preoccupy themselves with unanswerable questions. To rectify this, philosophy must have a method of determining whether or not a successful inquiry is possible for a given topic.
To this end, Hume recommends examining the components of a given concept. Specifically, one must comb through the concept’s contents for Humean ideas (i.e. thoughts about things) and then determine whether those ideas correspond to actual impressions (i.e. sensory observations of the real world). If at any point it is discovered that a concept does not correspond to any ideas or that the concept’s ideas do not correspond to any impressions, the concept is said to be unintelligible and the inquiry in which it is situated is is rendered futile.
There is, however, an exceptional case in which a concept can be validated without the help of impressions: namely, when its internal logic necessitates its truth, as in the classic proposition,
“All bachelors are married.”
In fact, these logically necessitated statements are the only universally true propositions, as their truth can be demonstrated regardless of circumstance. In contrast, when concepts do not possess this internal logic, but must be verified through empirical observation, their truth is conditional: it relies on the world being a certain way. For example, the statement, “Socrates is tall” is not a proposition whose truth can be known a priori (i.e. before experience). Instead, one would have to measure Socrates and compare his height to the average human height to determine whether the designation “tall” is appropriate.
This bifurcation of all propositions into “relations of ideas” (i.e. certain knowledge) and “matters of fact” (i.e. probable knowledge) is known as Hume’s Fork. As Hume says, if an idea contains neither “abstract reasoning concerning quantity or number” nor “experimental reasoning concerning matter of fact and existence,” it should be “committed to the flames: for it can contain nothing but sophistry and illusion.”
Hume, an avowed atheist, uses this principle to attack Christianity’s “first cause” and “intelligent design” arguments for God. He points out that both claims rest on unfounded assumptions: humans were not around to experience the universe’s beginnings, which renders any attempt to attribute the universe’s cause or design to God mere speculation.
Now, although Hume holds that relations of ideas and matters of fact are more reliable than propositions that do not correspond to logic or the world, he is yet skeptical about even their explanatory power with regard to causation. In perhaps his most famous philosophical contribution, Hume argues that just because a cause has historically produced an effect does not mean that it necessarily and always will. To give an example, the laws of nature could conceivably change, and an inversion of gravity could make objects suddenly fall upward instead of downward. While such a scenario is unlikely, nearly inconceivable, it is nonetheless possible. Since there is no guarantee that the future will be like the past, Hume asserts that all causal assumptions are probable rather than certain.
Nevertheless, in order to function in the world, we must assume that things in the future will continue to behave as they have in the past. The implication of this is that we base our causal inferences not on reason but on custom. In other words, our past experiences (i.e. impressions committed to memory) accustom us to associate certain causes with certain effects, for instance dropping my phone and expecting it to fall to the ground rather than the sky.
It is important to note that Hume does not hold all causal knowledge to be equally probable. Hume suggests three criteria for assessing confidence in the relation between a cause and an effect:
- Proximity in time: does the effect immediately follow the cause or is there a temporal gap between the two?
- Proximity in space: does the effect happen close to its supposed cause or far away from it?
- Necessary connection: is there some power or force relating the effect to its cause or is it arbitrary?
For instance, it is hard to imagine that a toilet flush caused a blackout, even if it happened right before the blackout. In contrast, it is easy to imagine that a downed power line caused the blackout, as the relationship between power lines (which supply electricity) and blackouts (which occur when the electricity supply is interrupted) is at least ostensibly understood.
As an ethicist, Hume is a sentimentalist, which means that he rejects the idea that humans are merely self-interested creatures. Instead, Hume holds that we naturally experience empathy, as evidenced in tendencies such as our interest in civil society, our ability to enjoy art, and our desire to be in communion with other people. When evaluating the morality of an action, we thus look at how it impacts not only ourselves, but others as well. We then approve or disapprove of it based on how “agreeable” and “useful” it is as a whole.
As a political theorist, Hume rejects Locke’s notion that private property is part of the natural order of things, as it cannot be found in nature or among other species. Instead, private property is said to be simply a social convention that humans have adopted because it is useful in times of scarcity. Interestingly, Hume suggests that in times of overabundance, private property is rendered useless and thus can be eliminated — a perspective that anticipates Marx’s materialist philosophy. More generally, Hume’s ethical framework foreshadows utilitarianism, which argues that the goodness of an action is a measure of how beneficial it is to people relative to other possible courses of action.
By affirming both skepticism and sentimentalism, Hume establishes himself as a humble philosopher who acknowledges the limits of human understanding as well as a man of moral conviction who is not afraid to make positive claims about what is good for humanity.