As conventionally conceived by philosophers of science, scientific explanation of a phenomenon was simply epistemic (concerning knowledge), and centered on the phenomenon's counterfactual derivability from initial conditions plus natural laws (Hempel's covering law model). Yet Salmon found causality ubiquitous in scientific explanation, which identifies not only natural laws (empirical regularities), but accounts for them via nature's structure and thereby involves the ontic (concerning reality), how the phenomenon "fits into the causal nexus" of the world (Salmon's causal/mechanical explanation). For instance, Boyle's law relates temperature, pressure, and volume of an ideal gas (epistemic), but this was later reduced to laws of statistical mechanics via average kinetic energy of colliding molecules composing the gas (ontic). Thus, Salmon finds scientific explanation to be not merely "nomological"—that is, lawlike—but rather ontological, or "causal/mechanical". Though asserting the primacy of causal/mechanical explanation, Salmon was vague as to how scientists can attain it. Still, consensus among philosophers of science is that causation is central in scientific explanation.