The absolute value of this quantity is undefined, and (for the purposes of thermodynamics) the state of "zero internal energy" can be chosen arbitrarily. However, by the law of conservation of energy, any infinitesimal increase M d U in the total internal energy M U must be matched by the net flow of heat energy d Q into the sample, plus any net mechanical energy provided to it by enclosure or surrounding medium on it. The latter is − − P d V , where d V is the change in the sample's volume in that infinitesimal step. Therefore