Foremost, Mill argued that deductive logic does not depend on intuition for its proof, but rather on internal consistency. The foundational assumptions or axioms of all sciences are based on experience. Even the shared scientific axiom that nature is uniform or law-like is proved through simple enumeration of confirming examples, that is, through induction. More specific causal explanations do no more than summarize necessary and sufficient conditions: A necessary condition is always present when the effect occurs; the effect is always present when a sufficient condition is present. For example, a bullet to the brain is sufficient to cause death in most cases, but it is not necessary because people die from other causes. Or, oxygen is necessary to cause fire, but it is not sufficient because fire requires friction and combustible material, as well as oxygen.