David Hume — Source: Allan Ramsay [Public domain], via Wikimedia Commons
David Hume (1711-1776) is Scotland’s greatest philosopher. As such, he is in the tradition of the great British Empiricists, who include John Locke (1632-1704) and George Berkeley (1685-1753). As an Empiricist, Hume believed that the source of all our knowledge about the world comes from the senses. Hume is opposed to the Rationalists, whose ranks include Plato (427?-347 B.C.E.) and Descartes (1596-1650). The Rationalists believe that the source of all our knowledge is reason.
According to Hume, there are two kinds of objects of human reason. The first is “relations of ideas,” which is the kind of knowledge that mathematics, including geometry, algebra, and arithmetic, have. This kind of knowledge includes, in Hume’s words, “every affirmation which is either intuitively or demonstratively certain.”
Examples of relations of ideas include such facts as 3 + 2 = 5 from arithmetic, and the Pythagorean Theorem (“the square of the hypotenuse is equal to the squares of the two sides”) from geometry. This kind of knowledge is absolutely certain and does not come from sense experience. Sentences of relations of ideas are today called analytic sentences.
The second kind of object of human reason is “matters of fact.” This kind of knowledge is completely empirical, being the knowledge that we gain by using our five senses: seeing, hearing, touching, tasting, and smelling. A sentence such as “The sun is shining” represents a matter of fact if you look at the sky and indeed see that the sun is shining.
These two kinds of knowledge are very different. Knowledge of “relations of ideas” is a priori knowledge, which means that we arrive at it by using our reason and not our senses. A priori means prior to experience. Knowledge of “matters of fact” is a posteriori knowledge, which means that we arrive at it by using our senses. A posteriori means after (posterior to) experience.
Hume believed that “relations of ideas” tell us nothing about the world. After all, in the physical universe, there are no perfect squares or triangles. In addition, no one has ever seen a “one” or a “two,” although you may have seen one apple or the number “1” (which is a symbol) written on a chalkboard. Therefore, Hume is an Empiricist because he believed that all our information about the world comes from the senses.
This leads us to a question. Can we extend our knowledge beyond mere relations of ideas and matters of fact? Relations of ideas give us facts about abstract ideas (for example, 2 + 2 = 4). Matters of fact give us facts about the world (the sky is blue today). Is there any way in which we can go beyond these kinds of knowledge?
That is something that science attempts to do. On the basis of empirical facts, scientists attempt to derive principles that can be used to extend our knowledge. For example, the scientists who are physicians have discovered the principle that diseases have causes; in other words, there is a reason why someone catches a disease.
For example, you may suffer from cavities because of eating too many sweets and not brushing often enough after meals. Here’s another example: a lack of iodine in a person’s diet may cause a goiter (a swelling of the thyroid gland).
Because of his belief in the principle of causality, Jonas Salk studied polio until he was able to invent a vaccine that would prevent people from being afflicted with polio.
Modern science is based largely on the principle of causality. If astronomers look at a planet and see that it wobbles, they may conclude that it is affected by the gravity of an unseen planet. They reason that the wobble (the effect) occurs because of the gravity of another planet (the cause). On the basis of the principle of causality, astronomers concluded that Pluto must exist before they ever observed it.
According to Hume, “All reasonings concerning matter of fact seem to be founded on the relation of cause and effect.” As a philosopher concerned with how we acquire knowledge, Hume decided to investigate the principle of causality. After all, according to Hume, only by reasoning about cause and effect can we “go beyond the evidence of our memory and senses.”
Hume therefore performs a thought experiment. He asks you to suppose that an intelligent person is suddenly brought into the world, then Hume asks, what would that person — at first — observe? Such a person, although intelligent, would simply observe objects and events. One event would happen, then another, then another, etc. But the person would not immediately be able to arrive at the concept of cause and effect.
Newborn infants, some of whom are very intelligent (just ask any mother), go through the same thing. Set an infant in front of a mirror for the first time and it won’t understand that it is looking at its own reflection.
After a while, that intelligent person will arrive at the concept of cause and effect. Why? Certainly not because the intelligent person has seen causality. Causality is not the kind of thing that anyone can see. All we see is one event happening, then another, etc. But no one has ever seen causality; that is, no one has ever seen a necessary connection between events.
Let’s say that our intelligent — and now experienced — person sees snow. Immediately he associates the idea of coldness with the snow. But why does he do that? According to Hume, “he has not, by all his experience, acquired any idea or knowledge of the secret power by which the one object produces the other, nor is it by any process of reasoning he is engaged to draw this inference ….”
So why does he associate snow with coldness? Hume’s answer is “custom or habit.” The intelligent person has experienced snow on a number of occasions, and each time the snow has been cold, and therefore by habit the person associates snow with coldness. But the intelligent person has never seen a necessary connection between snow and coldness.
What the intelligent person does see are these things:
1) Constant conjunction: Each time the intelligent person picks up a snowball, he or she feels a cold sensation, and
2) Temporal priority: First the intelligent person picks up a snowball, and then he or she feels a cold sensation.
However, the intelligent person does not see a necessary connection between snow and coldness — or between flame and heat, for that matter. Without a necessary connection, we can have no knowledge of causality.
According to Hume, all inferences from experience (for example, the inference that snow is cold) are “effects of custom, not of reasoning.” The intelligent person picks up one snowball, finds that it is cold, then picks up another snowball, finds that it is also cold, etc. After experiencing several snowballs, the intelligent person concludes that snowballs are cold.
The same thing applies to the game of pool. We use a cue stick to hit the cue ball against a colored ball. Through custom or habit, we expect the cue ball to hit a colored ball and make it move. But according to Hume, this is something we expect only through custom or habit. Our senses do not detect a necessary connection between the movement of the cue ball and the movement of a colored ball.
Instead, we hit a cue ball against a colored ball and the colored ball moves, then we hit a cue ball against another colored ball and once again the colored ball moves, etc. Eventually, we expect that whenever we hit a cue ball against a colored ball that the colored ball will move.
But is that the way reason works? According to Hume, no. A geometer does not work with one circle, then another circle, then yet another circle, etc., until he concludes something. No. All the geometer has to do is work with one circle. A theorem that applies to one circle will apply to all circles, and so there is no need for the geometer to work with many circles.
In Hume’s words, “The conclusions which [reason] draws considering one circle are the same which it would form upon surveying all the circles in the universe. But no man, having seen only one body move after being impelled by another, could infer that every other body will move after a like impulse. All inferences from reasoning, therefore, are effects of custom, not of reasoning.”
If Hume is right, his reasoning would have a great effect upon Humankind. By doing away with the principle of causality, he has removed the prop of modern science. Without the principle of causality, we would not know what would happen if we were to crack a chicken egg to make an omelet. The chicken egg might explode like a scene in a Sylvester Stallone movie; it might pour forth beautiful music, it might turn into a beautiful woman or an ugly prince, or a solid gold egg might be inside.
When a philosophical theory appears to contradict common sense, we need to take a close look at that philosophical theory. Hume has used empirical reasoning and shown that it denies that we can have knowledge of causes and effects. Perhaps by showing us this consequence of Empiricism, Hume has shown that Empiricism is incorrect. Perhaps reason does make a contribution to knowledge.
The philosopher Immanuel Kant (1724-1804) will attempt to forge a compromise between Empiricism and Rationalism. He will argue that both the senses and reason have a contribution to make to knowledge.
Note: The quotations by Hume that appear in this essay are from his book An Enquiry Concerning Human Understanding.
Copyright by Bruce D. Bruce; All Rights Reserved
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