Next, the two words "expiation" and "propitiation" present problems. It has been argued that the former, which means to purge away, needs to be distinguished from the latter, which means to appease a person, and that it is propitiation which presents the problem for those who are critical of the idea of penal substitution. Karl Barth (and later Jürgen Moltmann) argued in Church Dogmatics IV/1 that propitiation and expiation are false categories when applied to the triune God: If God forgives us in and through Christ ("Christ pays our debt"), then the cost has been borne by God in, as, and through Christ. For God to propitiate himself is expiation; because expiation is always self-propitiation as it means the forgiver paying the debt (here, the price of the sin) at his own expense. Hence Dietrich Bonhoeffer says grace is free, but is not cheap.