Hume's Empiricism: Relations of Ideas, Matters of Fact, and Critique of Metaphysics

Relations of Ideas and Matters of Fact: Our mental contents are reduced to impressions and ideas, which form the basis of our thought experiments.

1. Relations of ideas: These establish relationships between ideas and concepts. Their truth depends not on experience, but on the meaning of the terms. Relations of ideas are universal and necessary statements; their negation is contradictory and absurd.
2. Matters of fact: These establish relationships among facts that must be verified through observation and experience. These claims are contingent; their negation is possible. Matters of fact often refer to the future. Relations of ideas and matters of fact are the only two types of propositions that can provide knowledge.

Critique of Metaphysics: Hume discards three substances: external substance (bodies), infinite substance (God), and thinking substance (self). He asks: What is certain knowledge? To find an empiricist answer, he applies his principle and demonstrates that there is no certain knowledge of any of these three substances.

1. Critique of external substance: For Hume, our idea of substance is a primary objective unit, the support of qualities that cause our perceptions. Where does this idea of substance come from? Hume concludes that the idea of substance does not originate from any impression; therefore, it is not and cannot be considered valid—it is an illusion. What kind of impression would this idea come from? There is no continuity of any impression that we attribute to substance. It is a creation of our imagination that groups various impressions together over time. Hume recognizes the need for this belief to live normally.
2. Critique of the idea of infinite substance: Our idea of God is the infinite substance. Applying the empiricist principle, Hume asks what impression we may derive this idea of infinity from. The idea of infinite substance lacks legitimacy and must be considered invalid; therefore, there is no such knowledge.
3. Critique of the idea of thinking substance: The idea of self has not yet been questioned by any philosopher.