If, in a process of change of state of a closed system, the energy transfer is not under a practically zero temperature gradient, practically frictionless, and with nearly balanced forces, then the process is irreversible. Then the heat and work transfers may be difficult to calculate with high accuracy, although the simple equations for reversible processes still hold to a good approximation in the absence of composition changes. Importantly, the first law still holds and provides a check on the measurements and calculations of the work done irreversibly on the system, W A → → B p a t h P 1 , i r r e v e r s i b l e , and the heat transferred irreversibly to the system, Q A → → B p a t h P 1 , i r r e v e r s i b l e , which belong to the same particular process defined by its particular irreversible path, P 1 , through the space of thermodynamic states.