Philosopher David Hume on Sceptical Doubts Concerning the Operations of the Understanding Summary
SECTION IV: Sceptical Doubts concerning the Operations of the Understanding
21-21. There are two types of objects of human reason: Relations of Ideas and Matters of Fact. Relations of Ideas are the affirmations that are intuitively or demonstrably certain. Such is the theorem of Pythagoras (whose truth does not depend upon there even being a triangle) and the truth that three times five equals half of thirty. These propositions can be concluded by thought and are independent of all existing things in the universe.
Matters of Fact are ascertained in a different manner and can never imply a contradiction. That the sun will not rise tomorrow is no more a contradiction than that it will rise. Its falsehood cannot be demonstrated. It is worthwhile to inquire into the nature of evidence that gives the assurance of a matter of fact beyond the testimony of senses and memory.
22-24. It is the relation of cause and effect that is the foundation of all reasonings concerning Cause and Effect. By that relation alone we can go beyond the evidence of our senses and memory. Consider, for example, this matter of fact: a man believes that his friend is in France. If you ask him why he believes this, he will give you a reason that is another fact: perhaps a letter received from him or the knowledge of a promise. Someone finding a watch on a desert island would conclude that a person had once been there. All our reasonings concerning fact suppose that there is a connection between the fact and its cause. We hear a voice in the dark and we are assured of another person. Why? Because that is the sound made by a person.
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How do we arrive at the knowledge of cause and effect? Not by reasoning à priori; but, without exception, entirely from experience. Present a man with an entirely new object and he can examine it and apply reasoning, but he will be unable to discover any of its causes or effects. An Adam -- even with rational faculties -- if given water could not conclude that it could drown him or that a fire he was shown could burn him. Unassisted by experience, our reason can draw no inference concerning real existence or matter of fact.
This proposition is true: causes and effects are discoverable by experience,and not by reason. But because we have previously believed that reason, without experience, can discover effects, we believe that, if we suddenly appeared in this world and saw one billiard-ball hit another, through our reason alone we would not have to wait for the event of the movement of the second to believe that with certainty it will move.
25-27. But to convince you that all the laws of nature are known only through experience and past observation leading to custom, I must ask: can the mind otherwise proceed with this operation, that is, without experience? It cannot find the effect in the supposed cause by examining the cause because the effect is totally different from the cause. The motion of the second billiard-ball is distinct from that of the first. Lift a stone in the air, and you have no reason à priori to expect the stone to fall. Without your experience, your reason can offer an upward or any other direction of motion. When I see one billiard-ball moving towards a second, can I not conceive a hundred different effects to follow the cause?
Reason tells me that it is conceivable that both may come to rest, or the first will return in the direction from which it came, or that it may go in any of many possible directions, or even leapfrog over the second. No à priori reasoning can show preference for any of these possibilities. For every effect is a distinct event from its cause. To choose one event over another we must have the assistance of observation and experience.
This is why philosophers should not -- and rational ones have not -- assigned ultimate causes. Laws of motion are discovered by experience. À priori reason could never reveal an effect, much less show an inseparable connection with a cause.
28. Our reasonings concerning matter of fact are founded on the relation of cause and effect. The conclusions of that relation are founded on experience. What then is the foundation of all conclusions from experience? It is not reasoning or any process of the understanding.
29. But what is it? It is not our senses. They can inform us of the colour and consistency of bread but neither sense nor reason can inform us of the qualities that fit it for the nourishment of the body. There is no known connection between sensible qualities and secret powers. And there is no reason why past experience (eating bread and our body being nourished) should be extended to the future. Because bread nourished me at one time does not mean that it will do so at another time. It is not a necessary consequence.
For consider these two propositions:
I have found in the past that an object has brought a particular effect; and
I will find that other objects of like appearance will be attended by like effects.
These two propositions are different. I may expect the second to be true but there is no chain of reasoning to infer it. If I am wrong, you produce the reasoning for this judgments.
30. There are but two types of reasoning: (1) demonstrative reasoning concerning relations of ideas and (2) moral reasoning concerning matter of fact and existence. I may conceive of something that looks like snow, but tastes of salt and is hot like fire. I can conceive of trees flourishing in December and decaying in June. These are not contradictions and cannot be proven false by demonstration or à priori reasoning.
Our arguments from experience can go no further than to show that similar causes lead us to expect similar effects. But if this conclusion were formed by reason, we would expect all eggs to taste the same. And there is no process of reasoning which would draw a different conclusion from one instance from that which it infers from a hundred instances.
It is, then, only the aid of experience which allows one to draw an inference; but that inference is not a causal connection, for otherwise the causal connection could be inferred from the first appearance. It is from experience that when we eat bread we expect the effect of nourishment. But the experience is that particular objects of bread at particular times in the past have given this effect. But "to have found in the past help" and "to expect in the future help" is not a tautology. Such propositions are not in any way the same. The inference is neither intuitive nor demonstrative. No argument from experience can prove a resemblance from the past to the future. But as a philosopher I must not leave this with scepticism but inquire with further curiosity; otherwise I would be guilty of unpardonable arrogance for I would have left the argument without further investigation. Because I have not found a conclusion does not mean that a conclusion cannot be found. Infants and even animals learn from experience. But to say that a child learns not to touch a flame does not mean that it is a rational argument that has led it to this conclusion.