Understanding Hume: Knowledge, Causality, and Impressions
Hume’s Philosophy: Impressions, Ideas, and Causality
According to Hume, ideas (memory, imagination) result from the mind’s operation on data previously obtained from impressions. Impressions are images we hold in our minds to remember or contemplate. Sense impressions (simple, compound) are strong, while ideas are weaker images left by those impressions in our thoughts. An idea is not a direct presence but implies a memory and is not immediate.
Ideas can be simple (corresponding to simple impressions) or complex (corresponding to complex impressions or arising from operations on simple ideas). Hume uses this distinction to establish a criterion of truth that he employs throughout his work. This is the copy principle, which states: only true ideas originate from impressions. To determine if an idea is true, we must identify the impression from which it derives. The limit of our knowledge lies in our impressions.
Modes of Knowledge
Hume identifies two modes of knowledge:
- Relations of Ideas: These relationships are formulated with tautologies. This level is characteristic of formal sciences (arithmetic, algebra, and geometry). For example, the proposition “two plus two equals four” is necessarily true; it is impossible for “two plus two equals four” to be false. Such a proposition is analytic, its truth can be known a priori, and it can be discovered by a simple operation of understanding. This is the realm of non-contradiction.
- Matters of Fact: The truth of these propositions is only known ex post and cannot have any justification other than impressions. They represent the world of facts and are the domain of empirical sciences, moral philosophy, and the science of human behavior. These propositions are synthetic, meaning their truth cannot be determined solely by analyzing the subject; experience is required.
The criterion of truth for matters of fact is the relationship between impression and idea; only truth derived from impressions is valid. Most propositions we use daily fall into this category.
Analysis of Causality
Hume’s radical empiricism led him to classify elements of knowledge into impressions and ideas. To determine if an idea is true, we must identify the impression from which it originates. The limit of our knowledge lies in our impressions. Applying this approach to matters of fact, our knowledge is limited to two things: our current impressions (what we see or hear) and the memories of our past impressions. Therefore, we cannot have any knowledge about the future because we have no future impressions.
Elements of Causality
Hume identifies three elements commonly associated with causality:
- Contiguity: Spatial and temporal proximity between cause and effect.
- Priority: The cause precedes the effect (e.g., fire before smoke).
- Necessary Connection: The presumed connection between cause and effect.
Commonly, it is believed that a necessary connection exists between A and B. Hume critically examines this idea. The necessary connection is not obvious a priori; we know through the senses that A is followed by B, and that B has consistently followed A. However, this necessary connection is unprovable. We are simply accustomed to believing it due to the psychological mechanism of repetition. Hume argues that we do not know, but rather believe or suppose a causal relationship. Hume denies that causality is an inherent property of objects; it is merely a belief or habit. In reality, there might not be any antecedent and consequent relationship between A and B.