Robert Nozick's truth-tracking theory of knowledge is an externalist epistemological account that defines knowledge in terms of a belief's ability to "track the truth" across possible scenarios or worlds. According to Nozick, for a subject S to know a proposition P, four conditions must be met:

1. P is true.

2. S believes that P.

3. If P were not true, S would not believe that P. (Sensitivity condition)

4. If P were true, S would believe that P. (Adherence condition)

These last two conditions involve subjunctive or counterfactual conditionals that consider what would happen in nearby possible worlds where P is true or false. The sensitivity condition ensures that if P were false, the subject would not hold the belief that P, preventing lucky true beliefs from counting as knowledge. The adherence condition ensures that if P were true, the subject would indeed believe P, so the belief reliably tracks the truth123.

Nozick's theory replaces the traditional justification condition in the tripartite definition of knowledge (justified true belief) with these truth-tracking conditions. This allows the theory to avoid classic Gettier problems, where a belief is true and justified but only true by luck rather than properly connected to the truth16.

A key feature of Nozick's theory is that knowledge is not closed under known entailment, meaning that even if S knows P and knows that P entails Q, S may not know Q. This denial of epistemic closure arises because the sensitivity condition may fail for Q even if it holds for P. Nozick sees this as a virtue because it explains why skeptical hypotheses (like the brain-in-a-vat scenario) can undermine some knowledge claims without denying all knowledge12.

The theory also emphasizes the role of the method or way (M) by which the belief is formed, ensuring that the same method is used when considering whether the belief would be held in counterfactual situations. This addition helps avoid counterexamples where a subject might form beliefs by different methods in different possible worlds5.

In summary, Nozick's truth-tracking theory holds that knowledge requires a belief to be true, held by the subject, and sensitive to the truth in such a way that the belief would not be held if it were false and would be held if it were true. This externalist account focuses on the belief's relationship to truth across possible worlds rather than on the subject's internal justification136.

Key points of Nozick's truth-tracking theory:

• Knowledge requires truth, belief, and truth-tracking sensitivity and adherence.

• Sensitivity: If the proposition were false, the subject would not believe it.

• Adherence: If the proposition were true, the subject would believe it.

• Knowledge is not closed under known entailment (denies epistemic closure).

• The method of belief formation matters for tracking truth across possible worlds.

• Designed to solve Gettier problems by ruling out lucky true beliefs.

This theory is influential in contemporary epistemology for its innovative use of counterfactual conditions to analyze knowledge1236 (AI)