David Hume: Empiricism, Knowledge, and Causality
Hume’s Theory of Perceptions: Impressions and Ideas
David Hume categorizes perceptions (any mental content) into two types. Firstly, impressions are the direct feelings or sensations that occur because of a phenomenon; they come from our external senses. Secondly, we find ideas, which are internal representations within our mind, essentially thoughts. The primary difference between them lies in their degree of force and vivacity; impressions are stronger and more vivid than ideas.
According to Hume, ideas originate from impressions, meaning there are no innate ideas. All knowledge stems from experience. Impressions are produced directly in the presence of the phenomenon. An idea represents something in its absence. Ideas are divided into two types:
- Simple ideas: These cannot be decomposed further.
- Complex ideas: These are formed through the association of simple ideas.
Laws of Association of Ideas
The laws of association of ideas describe the principles by which the mind connects simple ideas to form complex ones. Hume identifies three main laws:
- Similarity: Connecting ideas that resemble each other.
- Spatio-temporal Contiguity: Connecting ideas of things that occur close together in space or time.
- Cause and Effect: Connecting ideas where one is perceived as the cause of the other.
Imagination and memory operate on these ideas.
Abstract Ideas: Nominalism vs. Empiricism
Abstract ideas are universal concepts (formed by abstraction), such as the idea of ‘man’. Hume argues they have no intrinsic value separate from specific impressions; an abstract idea is merely a more or less weak copy derived from impressions.
- Nominalism (Hume’s view): Universal abstract ideas like ‘man’ are just names used to refer to many similar individual instances. They are not derived directly from a single, specific impression but built through the association (similarity) of different perceptions of similar things. The idea is simply a name for things that resemble each other.
- Rationalism (Contrasting view): Rationalists might argue that we grasp the essence of ‘man’ through reason, and the word represents this essential idea.
For Hume (an empiricist), the mind forms the general idea by observing similarities among particular instances.
Hume’s Classification of Sciences
Hume distinguishes three types of sciences based on their subject matter and methods:
- Formal Sciences: (e.g., logic, mathematics) These are deductive sciences concerned with relations between ideas.
- Natural Sciences: (e.g., physics, biology) These are inductive sciences dealing with matters of fact.
- Social Sciences: (e.g., ethics, politics) These are practical sciences, also dealing largely with matters of fact but often with less precision.
This division echoes the distinction between truths of reason and truths of fact found in rationalism.
Formal Sciences
These are based on the principle of identity (A = A) and are essentially tautological. Their truths are necessary and do not require empirical verification (checking against experience). We can speak of absolute truth within these systems (e.g., in geometry or algebra). Formal sciences, while true, do not necessarily describe the empirical world; they concern the relations of ideas themselves.
Natural Sciences
These sciences do speak about the world and aim to provide new knowledge. They are based on experiences that can be checked (empirical data). They investigate relationships between real objects in space and time, often relying on past observations to make predictions. However, Hume raises a problem: how can we state general laws based only on past events? Any outcome remains logically possible. The statement “The sun will rise tomorrow” is based on past facts but remains only a probable affirmation, not a logical certainty.
Social Sciences
These practical sciences (like ethics and politics) are often based on less precise data than the natural sciences. Achieving the same level of accuracy is difficult. They rely on empirical data, but theories constructed within them are often considered more precarious. They seek useful knowledge perhaps more than absolute truth and can be seen as sub-disciplines relative to the natural sciences in terms of certainty.
Hume draws a significant distinction between formal and natural sciences. Formal sciences are based on the logic of reason. Natural sciences rely on the understanding, which involves applying reason to experience. For Hume, ‘reason’ alone (as in formal logic) can lead to certain knowledge about the relations of ideas, while ‘understanding’ involves a more practical application that requires both reason and experience.
Treatment of Space
Hume might distinguish two approaches to space:
- Pure Geometry: Analytic formulas where reason operates on numbers and abstract concepts.
- Descriptive Geometry: Reasoning about empirically observed space.
Critique of Causality and Induction
The natural sciences form the foundation of much of our knowledge about the world. They heavily rely on the principle of cause and effect (A causes B) to explain phenomena. This principle is fundamental both to scientific explanation and metaphysical assumptions.
Hume questions the rational basis of our belief in causality and, consequently, the certainty of natural science. He argues that our idea of causality arises from observing the constant conjunction of two events.
When two things consistently happen together, our mind associates them due to:
- Repetition
- Habit or Custom
- Belief
We observe one thing occurring after another, and through repeated experience, we develop a belief or expectation that the second will follow the first. We never directly experience a necessary connection, only the conjunction of events.
The Problem of Induction
Hume criticizes the principle of induction used in the natural sciences, which assumes the “uniformity of nature” – the belief that the future will resemble the past. He argues this principle itself cannot be proven logically or empirically; it’s a metaphysical assumption.
If we cannot rationally justify the belief that patterns observed in the past will hold in the future (uniformity of nature), then we cannot establish universal physical laws with certainty based on experience alone. This undermines both scientific claims to universal laws and metaphysical claims about first causes (like “Everything that begins to exist has a cause”).
Causal reasoning and induction are based on belief, which Hume suggests arises from habit and the human need to find order and explanation in the world. This belief is rooted in the imagination and the common ways our minds work, not in logical certainty.
Phenomenon
The term phenomenon refers to the aspect that things present to our senses; that is, the initial contact we have with things, what appears to us.