David Hume’s Fork is a logical model that pushes the boundaries of philosophical thought and questions the nature of knowledge.
David Hume’s Fork is a reasoning model from the 18th-century Scottish philosopher David Hume, a pioneer of the empiricist movement. (Empiricism: the view that knowledge can only be obtained through sensory experience.)
We often think we have ideas unsupported by our impressions, and Hume pondered the extent of this. To understand what he meant, we must keep in mind that there are only two types of propositions: demonstrative and probabilistic propositions, and that in our daily lives, we confuse the knowledge these two types of propositions convey.
After such a long-winded introduction, let’s get to Hume’s Fork:
The truth or falsehood of a demonstrative proposition is clear and evident. For instance, take the proposition “2+2=4”. Denying this proposition involves a logical contradiction. In other words, claiming that 2+2 does not equal 4 shows a misunderstanding of the meanings of 2 and 4. Demonstrative propositions in logic, mathematics, and deductive reasoning are known to be either true or a priori false, meaning they are independent of experience. However, the evidence for probabilistic propositions is not as clear, as they pertain to matters of empirical fact.
For example, a statement like “John is upstairs” is a probabilistic proposition because we need empirical evidence to determine its truth or falsehood. In other words, we can only verify or falsify it through experience, such as going upstairs to see if John is there.
With this knowledge, we can question whether any proposition is demonstrative or probabilistic. If it is neither, we cannot know whether it is true or false, and according to Hume, such a proposition is meaningless. This separation of all propositions into two distinct options is what is commonly known as “David Hume’s Fork.”
There is no issue in Hume’s reasoning up to this point, but things get interesting when he applies this logic to inductive inference—the ability to make inferences from past evidence. We observe a consistent pattern and implicitly conclude that it will continue in the future, assuming nature behaves uniformly. For example, we see the sun rise every morning and infer that it will rise again tomorrow. But is it really possible to claim that nature will follow this uniform model? To say the sun will rise tomorrow is not a demonstrative proposition, as denying it does not involve a logical contradiction. But it is not a probabilistic proposition either, because we cannot experience future sunrises.
The same problem applies to the statement “Event A causes Event B.” Although we can seemingly verify this, it is also open to scrutiny. It is logically consistent to say that A causes B, but it is not empirically provable because we cannot verify that B follows A in every case, making it not a probabilistic proposition.
As a result, Hume attributes such uniform causality and inductive reasoning to human mental habits.
At the beginning of my essay, I mentioned that Hume was a pioneer of the empiricist movement. His strongest stance against rationalism is that, according to him, we arrive at knowledge not through reason but through beliefs that stem from the habits and experiences we have internalized.
To summarize and illustrate this reasoning with a simple example: our belief that the sun will rise tomorrow or that juice will flow from the faucet instead of water, according to Hume, is not based on logic. It is the result of our conditioning, which teaches us that the world will be the same tomorrow as it is today.