In 1854 Clausius proved the famous theorem that bears his name by assuming the second "law" of thermodynamics. In the present paper we give a proof that requires no such assumption. Our proof rests on the laws of mechanics, a T-mixing property, an ergodic consistency condition, and on the axiom of causality. Our result relies on some recently derived theorems, such as the Evans-Searles and the Crooks fluctuation theorems and the recently discovered relaxation and dissipation theorems.