A passage in Or ha-Shem [Light of the Lord], the major work of Ḥasdai Crescas (1340-1410/11), the Jewish critic of medieval Aristotelianism, is adduced here as contributing to Spinoza's decisive step beyond the Aristotelian doctrine of God, namely, the integration of the attribute of extension into the essence of God's infinite being. According to Crescas place, correctly conceived, is void. Many among the ancients, he argues, identified a thing's place with its form. Although this identification is incompatible with his ontology, he uses it to equate God, metaphorically, with the infinite void that, on account of the identification made by many among the ancients, constitutes the form of the world. Metaphorically, therefore, God can be described as active infinite extension that produces, individuates, and determines the bodies of things in the same way as God, according to Crescas' ontology, literally produces, individuates, and determines their forms. The ontological constraints that prevented Crescas from actually identifying God with active infinite extension no longer bound Spinoza, who adopted a dualistic ontology of thought and extension derived from Descartes. I argue that Spinoza's conception of the attribute of extension is modeled on his conception of the attribute of thought, which in turn is related in important ways to the conception of the divine intellect in the medieval Aristotelian tradition. Spinoza's decisive step beyond this tradition is to expand the ontological scope of God's activity from one confined to thought to one that includes both thought and extension. The philosophical payoff for Spinoza's conversion of active infinite extension into one of God's attributes is the solution of an ontological problem common, he believed, to medieval Aristotelians, Crescas, and Descartes: how can God, conceived as active and incorporeal, be causally related to the world, conceived as passive and corporeal?