Many definitions in the sciences are modal, meaning, they deal with possibility. Let me give some examples. In physics, there is the definition of energy as the ability to do work. In chemistry, there is the definition of flammable, which means able to burn. In biology, there is the definition of two animals being of the same species, which means they are able to mate, not that they actually will mate. But suppose someone does not believe in non-actual possibilities, they believe the world can't be anything other than what it actually is. To such a person, many, perhaps even most scientific definitions would be unintelligible. Now, such a person can bite the bullet and believe that most scientific definitions are, strictly speaking, vacuous, even if useful. But is there some way to rescue or reformulate these scientific definitions so that our intuitions can be salvaged, without the ontological extravagance of positing possibility or counterfactuals? Or is there simply no such way to rescue those modal definitions from necessitarianism?

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To rescue modal scientific definitions from necessitarianism, one approach is to reinterpret them in terms of nomic necessity rather than metaphysical modality. Nomic necessity refers to the laws and regularities that govern a particular domain of inquiry. For example, the definition of energy as the ability to do work can be understood as stating a nomic necessity: energy is always capable of doing work because it is a property that arises from the fundamental laws of physics.

Similarly, the definition of flammability can be seen as stating a nomic necessity: combustible substances will always burn when exposed to heat and oxygen because it is their nature to do so according to the laws of chemistry.

Another approach is to reject modal definitions altogether and adopt a different framework for understanding scientific explanations. One alternative is to use dispositional essentialism, which views properties and relations as dispositions to behave in certain ways under certain conditions. This approach emphasizes the inherent tendencies and capacities of objects and substances rather than their potentiality or actuality. For instance, instead of defining an object as flammable, we could describe it as having the disposition to burn easily when ignited, based on its chemical composition and structure.

A third strategy is to embrace the ontological implications of necessitarianism and reject the idea that possibility and modality are fundamental aspects of reality. Instead, we could accept that everything that exists must exist and that there is only one possible world. On this view, scientific explanations would focus on describing the necessary structures and processes of that one world, without appeal to modal concepts.

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