The Relevant Alternatives Theory

Core Idea

Lewis's relevant alternatives theory proposes that to know something, one must be able to rule out relevant alternatives, but not all metaphysically possible alternatives. Relevance is context-dependent: what counts as a relevant alternative shifts with conversational context, explaining how everyday knowledge claims can be true while extreme skeptical hypotheses technically remain possibilities.

How It's Best Learned

Distinguish relevant from irrelevant alternatives in concrete cases. Understand how context determines which alternatives are relevant—in a context where only physical differences matter, mental simulation hypotheses aren't relevant. Apply this to explain why we typically say we know things despite skeptical possibilities.

Common Misconceptions

• The theory doesn't claim we can actually rule out all alternatives; it claims we can rule out the relevant ones. - Relevance isn't purely objective; it depends on conversational context. - The theory doesn't entail that knowledge claims are always relative or subjective, just context-sensitive.

Explainer

You know the structure of Cartesian skepticism from your study of external-world skepticism: a skeptic challenges everyday knowledge by pointing to possibilities you cannot rule out — you could be a brain in a vat, an evil demon could be deceiving you, your experiences might be a perfect simulation. The argument has two premises: first, you cannot distinguish the normal situation from the skeptical one; second, knowing p requires being able to rule out alternatives to p. Together these imply that you don't know ordinary things. The relevant alternatives theory attacks the second premise rather than the first.

Fred Dretske and David Lewis independently proposed that knowing p requires ruling out only *relevant* alternatives — not every metaphysically possible alternative. What counts as relevant is not fixed by logic but determined by context. In an ordinary conversation about whether there's a barn in the field, fake barns hidden in surrounding fields are not a relevant alternative (unless you happen to be in a county famous for fake barns). You know there's a barn because you can rule out everything that's relevantly in play. The brain-in-a-vat scenario is not relevantly in play in that context — it is too remote, too disconnected from any practical or conversational purpose, to count as an alternative you need to exclude in order to know.

The significant philosophical payoff is that this dissolves the apparent contradiction between ordinary knowledge and skeptical puzzles. When a philosopher explicitly raises the skeptical hypothesis in conversation, the context shifts — that once-irrelevant alternative becomes salient and thus relevant within the new conversational context. Suddenly "I know there's a barn" becomes harder to assert, not because your epistemic position changed, but because the standard for what counts as "ruling out enough" has shifted. This insight drives epistemic contextualism: the sentence "S knows that p" can be true in one conversational context and false in another without any change in S's evidence or cognitive situation. The standards encoded in "knows" are context-sensitive, much as the standards for "tall" vary with context. The relevant alternatives theory is thus both a theory of what knowledge requires and a framework for why we can speak truly about everyday knowledge while remaining technically vulnerable to philosophical skepticism — the skeptic changes the rules by raising the stakes, not by discovering that we were always deceived.