Relationalism about space faces well-known objections if it is limited to relations between actual bodies. These problems might be avoided through so-called modal relationalism, on which the relevant relata include possible entities. Leibniz is considered a founder of modal relationalism, appealing to relations among possible situations. This article argues that for the central type of relation in question, namely congruence, Leibniz cannot give an adequate basis for modal relationalism. This is because his criteria for determining congruence rely on actual perception, and because attempts to modalize this perception requirement conflict with his other commitments.
