 One philosophical debate where these worries over naturalism become particularly acute is the issue of the application of parsimony principles to abstract objects. The scientific data is—in an important sense—ambiguous. Applications of Occam’s Razor in science are always to concrete, causally efficacious entities, whether land-bridges, unicorns, or the luminiferous ether. Perhaps scientists apply an unrestricted version of Occam’s Razor to that portion of reality in which they are interested, namely the concrete, causal, spatiotemporal world. Or perhaps scientists apply a ‘concretized’ version of Occam’s Razor unrestrictedly. Which is the case? The answer determines which general philosophical principle we end up with: ought we to avoid the multiplication of objects of whatever kind, or merely the multiplication of concrete objects? The distinction here is crucial for a number of central philosophical debates. Unrestricted Occam’s Razor favors monism over dualism, and nominalism over platonism. By contrast, ‘concretized’ Occam’s Razor has no bearing on these debates, since the extra entities in each case are not concrete.