The common law analyses crime into two elements. The actus reus is the external element — the prohibited act, omission or state of affairs. The mens rea is the fault element — the intention, knowledge, recklessness or wilful blindness the offence requires. Conviction requires the prosecution to prove both, coinciding in time.
The pairing embodies a moral claim: punishment attaches to choice, not accident. Offences differ in the fault they demand — murder requires more than manslaughter, and regulatory offences may need only negligence or, exceptionally, no fault at all — and grading that hierarchy is much of what criminal law does.
Canadian law adds a constitutional floor: under s. 7 of the Charter, crimes carrying imprisonment require a fault element, and the stigma of murder requires subjective foresight of death (Vaillancourt, Martineau).