The minimum requirement for criminal liability is the performance by a person of conduct which includes a voluntary act or the omission to perform an act which he is physically capable of performing. If that conduct is all that is required for commission of a particular offense, or if an offense or some material element thereof does not require a culpable mental state on the part of the actor, the offense is one of "strict liability". If a culpable mental state on the part of the actor is required with respect to any material element of an offense, the offense is one of "mental culpability".