1 This is the penultimate draft of the paper that is forthcoming in “Metaphilosophy” Vol. 49, Nos. 1–2, January 2018 ON QUANTITATIVE AND QUALITATIVE PARSIMONY MACIEJ SENDŁAK 1. Introduction When evaluating philosophical and nonphilosophical theories, one often focuses on their methodological virtues. * One of these virtues is simplicity, which is usually associated with the principle of parsimony, known as Occam’s Razor (OR). The principle itself has been put forth in various ways, but the most common formulation states: (OR) Entities should not be multiplied beyond necessity. 1 Philosophers often refer to OR as the principle of economy of explanation, since according to this principle one should not believe in the existence of more entities than are necessary to explain a given phenomenon. 2 Because of this, when comparing two explanations (theories) of the same data, preference should be given to the one that is committed to a parsimonious (or simpler) ontology. Thanks to this we choose the more effective theory: that is, the one that allows us to explain phenomena without postulating superfluous entities. 3 Although the principle itself is not a novelty, philosophical investigations in the twentieth century have shed new light on it. This includes the results of observations made by Willard van Orman Quine and David Kellogg Lewis. Quine drew a distinction between ideological and ontological parsimony (Quine 1951), whereas Lewis showed that there is an important difference between a quantitative and a qualitative reading of OR (Lewis 1973). 4 The Lewis distinction is supposed to allow him to dismiss one of the charges against his theory of a plenitude of real, existing, possible worlds. * Earlier versions of this material were presented in Warsaw (University of Warsaw) at “The Seminar of the Department of Analytical Philosophy” and in Cracow (Jagiellonian University) at “The 2 nd Cracow Workshops in Analytical Philosophy.” I am grateful to the participants of these meetings for their helpful comments and discussions, especially to Tadeusz Ciecierski, Christopher Daly and Björn Lundgren. I would also like to thank to the anonymous reviewers for this journal for their comments concerning the earlier versions of the paper. This material is based on the work supported by The Foundation for Polish Science under program “START 2017.”