Causality, Probability, and Substance: A Humean Inquiry

The Problem of Causality

The cause-effect connection, a law of association of ideas, links events in the world. Hume, rejecting dogmatic positions, examines the legitimacy of this idea using the empiricist principle of “copy.” Any idea or belief must have a corresponding impression. If no impression is found, the idea is illegitimate. Hume applies this principle to causality and finds no necessary connection between phenomenon A (cause) and phenomenon B (effect). We only observe one event followed by another, not a necessary link. Hume concludes that the idea of necessary connection is a product of our imagination. Repeated observation leads us to believe a connection will always exist, forming a habit. This projection of past into future is practically useful, making the world predictable. However, Hume emphasizes that habit only yields beliefs, not universal knowledge.

The Probability of Science

The questioning of causality impacts science. Scientific explanations, descriptions, and predictions often rely on causal relationships. If causality is merely a habit of thought, science becomes a system of beliefs, not universal laws. Scientific statements are beliefs supported by custom and tradition. Repeated observation strengthens our belief, but these remain beliefs, not universal truths. They are, at most, probable.

Criticism of the Idea of Substance

Metaphysics, particularly since Descartes, focuses on the idea of substance: extended (world), infinite (God), and thinking (soul). Hume critiques this traditional view.

Critique of Extended Substance

Locke deemed substance unknowable, and Berkeley denied matter’s existence. Hume argues our idea of substance is an objective reality supporting qualities that cause our perceptions, with permanence exceeding theirs. But what impression underlies this idea? We only perceive qualities (smell, color, shape), not an underlying entity. Following empiricist principles, lacking a corresponding impression, the idea of substance is invalid—an illusion. Our impressions are discontinuous, while substance is perceived as continuous. This perceived continuity (e.g., a rose remaining when unseen) leads us to believe in substance. However, no continuous impression supports this. The idea of substance is a useful, yet unjustified, belief, enabling us to navigate a seemingly permanent world.

Critique of Infinite Substance

Our idea of God is an infinite substance with perfections (omniscience, omnipresence, etc.). Applying Hume’s criterion, what impressions support this? Our perceptions are specific and concrete, making an impression of infinity unlikely. Thus, the idea of an infinite, perfect substance lacks a corresponding impression, rendering theological or metaphysical knowledge of God invalid. Hume acknowledges the natural and necessary human tendency towards religious belief.

Critique of Thinking Substance

Hume challenges the unquestioned idea of the “I.” Traditionally, the “I” is the subject of perceptions, distinct from them. But what impression underlies it? None. The self is assumed stable beneath changing perceptions, forming our personality’s core. Hume questions this, arguing the idea of self, like other substances, is a belief—a product of imagination—imposing permanence and continuity onto something discontinuous.