How do we know which food is best for us? We might start a low-carb diet. Then we switch to whole grains, or even go fully vegan—only to return to a low-carb diet yet again. We constantly change our minds.
Even scientists keep revising their perspectives. Why is it so difficult to be certain on issues like these?
The Scottish philosopher David Hume answered this question almost 300 years ago.
Premise: We know everything from perception
As an empiricist, David Hume believed that our minds are initially ’empty’. All our knowledge comes from outside, we must observe the external world.
This idea is challenged by rationalists, who argue that our ability to know things comes from inside.
They believe we can gain knowledge through reason, logic, and mathematics.
This is important!
While rationalists like René Descartes mistrust our senses and seek enduring intellectual truths, empiricists like David Hume focus on observation. The ever-changing physical environment is their main source of knowledge.
Does the material world have a structure?
Empiricists trust that this material world around us is not just random and chaotic. What is at the core of this trust? The belief that everything was caused by something else. For example, an apple falls from a tree because of gravity.
The concept of cause and effect holds everything together. Everything is caused by something, which was in turn caused by something else… and so on. This idea is the universal glue of our material world.
Hume wanted to know whether this trust in cause and effect was justified.
Cause & effect
He started with a clarification of what the idea of cause and effect means:
Two events must be close to each other in space and time (contiguity). One of them, the cause, must come before the other, the effect (temporal priority). And most importantly: This succession of events must always happen (constant conjunction).
These three characteristics let us examine the past and predict the future! For example, when we see a human footprint in the sand, we can assume that somebody has walked there in the past.
Likewise, before we drop an egg on the floor, we can predict that it will break in the future.
But are these kinds of assumptions necessarily true?
Hume’s famous reply was: No.
The problem of Induction
Here is his explanation: Imagine you witness one billiard ball hit another. You see the first ball moving—then you hear a smacking sound. The first ball stops to a halt, while the second ball starts to move. Why did the second ball move? Because it was hit by the first one. We assume a necessary connection.
Now Hume says, it is thinkable that something else might have happened after the collision.
So before our observation there is no logical certainty for what might happen. What about after our observation? Did we really see that one event caused another? No. We saw that the movement of the second ball happened after the collision, that’s all. We could not observe the necessary connection between these two events itself.
So why do we assume that one event caused the other?
Because of habit.
Whenever we see events happen one after another often enough, we believe there’s a connection. This is not a rational justification! But a little caution is in order. Hume does not doubt that everything is connected by cause and effect. He just points out that this is an assumption based on habit, not on compelling rational logic.
Deduction vs. Induction
It is very important that we understand the difference between two kinds of reasoning:
We start with general principles and draw specific conclusions. For example, we can argue: All dogs can bark. Freddy is a dog. So, Freddy can bark.
This sort of argument is called a deduction. Deductions provide logical certainty. If the premises are true, then the conclusion must also be true. Deductions are usually not very exciting, because they more or less reveal how we defined something in the first place.
A second kind of argument starts with specific examples and guesses a general rule. For example: The sun has risen every day so far. Therefore, the sun will probably rise tomorrow.
This kind of argument is called induction. It is based on probability, not on certainty.
In short, deductions move from general rules to specific cases with certainty, while inductions move from specific cases to general rules with probability.
When we observe phenomena in the material world to discover rules or ‘natural laws’, we use inductive reasoning. Hume’s discovery was that this kind of inductive thinking is not based on reason but on a feeling that the future resembles the past. As a result, every scientific theory we develop is just that: a theory.
Why this is useful
This is also why we have to change our mind so often. Our reasoning about proper nutrition for example is inductive! We make observations repeatedly and assume rules about what works and what doesn’t. These rules can be good, but never absolutely certain. If they don’t work, we change them.
Our habit to assume causal connections when we observe things repeatedly can also explain our inclination to superstition or to accept lies if they are repeated often enough. Scientific inductive statements must always be examined carefully and, if necessary, corrected. This is the difference between just “following” the science and actually doing it.
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