Abstract. We examine the possibility of applying knowledge representation and automated reasoning in the context of philosophical ontology. For this purpose, we use the axioms and propositions in the first book of Spinoza's Ethics as knowledge base and a tableau-based satisfiability tester as reasoner. We are able to reconstruct most of Spinoza's system with formal logic, but this requires additional axioms which are assumed implicitly by Spinoza. This study illustrates how tools developed in computer science can be of practical use for philosophy.