Hume on Knowledge, Causality, and Moral Sentiment
Hume: Impressions, Ideas, and Knowledge Sources
For David Hume, sense experience is the ultimate source of our knowledge; he argued against the existence of innate ideas. He referred to anything that enters our mind through the senses as a perception. Hume distinguished between two kinds of perceptions: impressions and ideas.
- Impressions: Perceptions that enter the mind with greater force and vivacity.
- Ideas: Fainter images or copies of impressions, used in thinking, reasoning, and imagining.
The impression is the original, and the idea is its copy. All legitimate ideas must ultimately relate back to their corresponding impressions. If an idea cannot be traced to an impression, Hume considered it illegitimate. There are two types of impressions: impressions of sensation (from external senses) and impressions of reflection (from inner feelings and emotions). Impressions of reflection often arise from initial impressions of sensation. Ideas, in turn, can be derived from both types of impressions.
The Association of Ideas in Hume’s Thought
The association of ideas is a central theme for Hume, governing the functioning of thought. This mental process brings together and combines ideas according to specific principles or ‘laws’. Hume identified three primary laws of association:
- Similarity: Ideas resemble one another.
- Contiguity in Time or Place: Ideas of things that occur close together in space or time.
- Cause and Effect (Causality): Ideas related as cause and effect.
These laws operate on our impressions and ideas, not on the objects themselves. It is a mental habit or custom that leads us to relate ideas in these ways. Hume suggested that causality might be reducible to the regular succession of events (contiguity), and similarity involves comparing ideas, potentially leaving contiguity as a fundamental principle.
Hume’s Fork: Relations of Ideas vs. Matters of Fact
Hume divided all objects of human knowledge into two categories:
- Relations of Ideas: Found in formal sciences like geometry and algebra. They are discoverable by thought alone, provide demonstrative certainty, and their denial implies a contradiction (e.g., ‘a square has four sides’).
- Matters of Fact: Deal with existence and occurrences in the world. Knowledge of them arises entirely from experience. There is no logical contradiction in denying a matter of fact (e.g., ‘the sun will rise tomorrow’ is conceivable). Certainty about matters of fact comes from experience, not logical proof.
Experience leads to habit or custom, which forms the basis of our beliefs about matters of fact. This habit creates an expectation or belief that patterns observed in the past will continue in the future.
Hume’s Critique of Causation
The concept of causation is critically important in Hume’s philosophy. He observed that our idea of causation typically involves three components:
- Contiguity in time and place: The cause and effect occur close to each other.
- Priority in time: The cause precedes the effect.
- Constant conjunction: The two events have always been observed together in the past.
However, Hume argued that the supposed necessary connection between cause and effect is problematic. We have no distinct sensory impression of this necessary link itself. From experience, we observe contiguity and temporal succession, but not the necessity of the connection. Therefore, the principle of causality is a mental association arising from habit or custom based on repeated experience. We observe contiguity and succession, but we cannot empirically affirm a necessary causal relationship in reality. When we speak of causality, we describe a pattern in our thinking, shaped by habit.
Hume’s Skepticism Towards Metaphysics
Following his empiricist principles, Hume was critical of traditional metaphysics. Any abstract idea that cannot be traced back to a corresponding impression is deemed illegitimate. His denial of a discoverable necessary connection in causality undermines metaphysical claims about real causal powers. Similarly, the idea of substance is criticized as merely a collection of simple ideas united by the imagination, not corresponding to any specific impression. Hume’s philosophy challenges metaphysical concepts like God and the soul, denying the possibility of traditional metaphysical knowledge. While he accepts mathematics (as relations of ideas) and experimental sciences (as useful investigations of matters of fact), he grounds the latter in probability based on custom and habit, not absolute certainty.
Hume’s Moral Philosophy: Sentiment and Utility
Hume investigated the foundation of morality, grounding it in human nature and sentiment, not reason. Moral distinctions (good and bad) arise from feelings of approval or disapproval evoked by actions or character traits. Often, what is considered good aligns with what is useful or agreeable, either to the individual or to society. There is a natural human moral sentiment that leads us to approve of virtues (often linked to utility) and disapprove of vices. The judgment of an action’s morality is determined by a pleasant feeling of approval or an unpleasant feeling of disapproval it evokes.