In semantics for conditional logic using possible worlds, a condition that for any conditional $\phi \to \psi$ in which $\phi$ is consistent, of all the possible worlds at which $\phi$ is true there exists a world or set of worlds more similar to a world $w$ than any other, i.e., at least one world minimally dissimilar to $w$. Coined by the philosopher Robert Stalnaker (1940– ), the limit assumption received a difficult criticism from philosopher David Lewis (1941–2001) by suggesting that with respect to a one inch line $l$, there is no closest world at which ‘$l$ is longer than one inch’ is true.

In accounts for conditional logics with a system of spheres semantics, the limit assumption is represented as the constraint that, for any world’s system of spheres ${\text{\$}}_w$,

• • there exists a sphere $S_i\in {\text{\$}}_w$ such that for all $S_j\in {\text{\$}}_w$, $S_i\subseteq S_j$

That is, there exists a smallest sphere $S_i$ containing the worlds most similar to $w$.