David Hume’s Empiricism: Causality, Ideas, and Critique
Hume: Causality & Ideas Not From Reason
Why is the principle of causality not knowledge derived from relations of ideas? This knowledge is not a truth achieved by reason either intuitively or demonstratively. Reason cannot intuitively link cause and effect, as they are distinct and separable ideas. Furthermore, this knowledge cannot be reached via demonstration because the opposite (i.e., an effect not following its supposed cause) does not involve any logical contradiction or absurdity. The mind finds the absence of causal relationships, or the inability to discover them, as conceivable as their existence and our potential knowledge of them.
Critiques: Hume’s Probable Causality
Criticisms of Hume’s defense of probable causal relationships between perceptions include:
- From a skeptical standpoint, we cannot establish something as probably true if we do not know what is definitively true. Without knowing truth, we cannot determine if we are more or less close to it.
- Experience cannot provide a basis for probability: the number of observed cases divided by an infinite number of unobserved cases yields a probability that tends to zero.
- If we begin with the certainty that the future will probably resemble the present and the past, this initial certainty is absolute. However, experience does not provide absolute certainties; such certainties would have to be a priori. This leads to an a priori assumption. Alternatively, if this certainty is derived from previous experience, as Hume suggests, then that previous experience must have been built on a prior certainty, leading to an infinite regress.
Hume: Simple Ideas from Simple Impressions
Hume argues that simple ideas are derived from simple impressions. His evidence is that if an individual lacks the capacity to experience certain impressions (e.g., a blind person regarding colors), or has not experienced a particular impression, the corresponding ideas will never form in their mind. He notes an exception: an individual might conceive of a missing shade of a color if presented with a spectrum of other shades, even without having had a direct impression of that specific missing shade. However, Hume considers this exception too insignificant to alter his general principle that all simple ideas are derived from simple impressions.
Hume’s Knife: Empiricism & Metaphysics
What is meant by “Hume’s Knife,” and how does it compare with logical positivism and Popper?
The Principle of Hume’s Knife Explained
Hume’s Knife is a methodological principle, reminiscent of Ockham’s Razor. It states that if an idea cannot be traced back to its constituent simple impressions, the term designating that idea lacks meaning. Hume used this criterion to challenge metaphysical speculations. In effect, Hume introduces a sharp criterion for assessing the validity of our ideas. To determine if an idea is meaningful, we must check if it originates from some impression. If we can trace it to an impression, it is a genuine idea; otherwise, it is a fiction. Therefore, the limits of our knowledge are set by our impressions. Hume shares the fundamental premise of empiricism with Locke and Berkeley—that experience is the source of all knowledge—but, unlike them, he also emphasizes that experience sets the limit of our knowledge.
Hume’s Knife and Logical Positivism
This approach is similar to the criteria of demarcation and meaning later studied by logical positivists. Neopositivists later developed criteria of demarcation (such as verifiability or falsifiability) to distinguish scientific statements from metaphysical ones, and also criteria of significance. For neopositivists, following Hume’s line of thought, metaphysical propositions would lack cognitive significance.
Hume’s Knife Contrasted with Popper
Karl Popper, however, distinguished his criterion of demarcation (falsifiability) between science and metaphysics from the criterion of significance. For Popper, metaphysical propositions, while not scientific, can still be meaningful and play an important role in the context of discovering scientific theories.
Hume on Berkeley’s View of General Ideas
What great contribution, in Hume’s view, did Berkeley make to philosophy? According to Hume, Berkeley made one of the most significant and valuable discoveries: that general ideas are actually particular ideas associated with a general term, which gives them a broader application.