 Traditionally, philosophers of science have typically focused on theories, which they assumed were sets of natural laws (Hempel 1966) (see entry on  laws of nature).  Examples of laws of nature include the following. Newton’s universal law of gravitation is \(F_{ab} = G(m_a m_b)/r^2\) which states that the gravitational force between two objects \(a\) and \(b\) is proportional to their masses and inversely proportional the square of their distance. Newton’s second law of motion is \(\mathbf{F} = m\mathbf{a}\) where the vector sum of forces \(\mathbf{F}\) equals mass \(m\) times acceleration vector \(\mathbf{a}\). Finally, there is the Boyle-Charles ideal gas law which is \(PV = nRT\) which says that the pressure \(P\) and volume \(V\) of a gas is proportional to its temperature \(T\). Philosophers of science and metaphysicians debate what makes statements like those above laws. The answers include generalizations concerning empirical regularities (Ayer 1956), relations between universals (Dretske 1977), the best system that trades simplicity in primitives for explanatory power (Lewis 1983), encode various symmetries (Fraassen 1989), and so on. One powerful worry with thinking that theories are sets of laws of nature is that if, at a minimum, laws are true universal generalizations, then there no such laws at least in the special sciences since those generalizations have exceptions (Beatty 1995; Cartwright 1983). That is, they are false. As the result of this argument amongst others, philosophers of science have increasingly focused on models (Giere 2010; Odenbaugh 2005; Weisberg 2012) (see entry on  models in science).