Which is the second rule of H. H. Gossen?

Gossen's First Law is the "law" of diminishing marginal utility: that marginal utilities are diminishing across the ranges relevant to decision-making.
Gossen's Second Law, which presumes that utility is at least weakly quantified, is that in equilibrium an agent will allocate expenditures so that the ratio of marginal utility to price (marginal cost of acquisition) is equal across all goods and services.                                                           
Gossen's Third Law is that scarcity is a precondition for economic value.