A tracking theory of knowledge is one that describes knowledge as a belief that tracks the truth in a reliable way.
Nozick describes four conditions for how a person, S, can have some knowledge of a proposition, P. In order for S to know P, Nozic says that these conditions must be met:
Nozick's definition is known as a truth-tracking one. Knowledge is such because it tracks the truth — justification of a belief is only valid insofar as it reliably keeps track of what is true.
In the truth-tracking theory, Nozick makes use of the subjunctive conditional (or counterfactual conditional), a non–truth-functional logical operator that differs from the normal material conditional (→ or ⊃). A subjunctive conditional, sometimes symbolized as ‘’ or ‘>’, is used to formalize instances where a conditional operation is meant to intend that if something were the case, something else would result.
Subjunctive conditionals are generally interpereted as if they were modal operators, using possible world semantics. Thus, the statement P Q is meant to imply that if P is true in some close, or similar, possible world, then Q is also true in that world.
In metaphysical and epistemological talk, a close possible world is intended to be a world that is particularly similar to the present one, with some details (especially the truth of P) changed.