"Methodological naturalism requires scientific theories to mention only natural things. One problem with this suggestion is that scientists are constantly postulating new entities, such as quantum wavefunctions, quarks, and genes. And who is to say whether or not these entities are natural? What are the defining characteristics of natural entities? The problem before us, then, is to complete the following definition:NAT: x is natural just in case…But it would be extremely difficult to complete this definition in a way that would be useful for guiding scientific practice. First, it wouldn’t be helpful to define natural entities as those mentioned by our current best scientific theories. This is because methodological naturalism would then lead to extreme conservatism about ontology: no new entities should be introduced in science. Second, it wouldn’t be helpful to define “x is natural” in terms of the words “natural,” “supernatural,” or any synonym thereof. [...] Third, it wouldn’t be helpful to define “x is natural” in terms of space, time, energy, or mass. Contemporary science already defies simple intuitions about what is natural, and we can expect future science to do so to an even greater extent."