The paper discusses the concept of adequacy central for Pertażycki’s methodology. According to Petrażycki any valuable scientific theory should be adequate, that is, neither limping (to broad with respect its actual scope) nor jumping (too narrow with respect to its actual scope). Consequently, adequacy of a theory is a stronger condition than its truth. Every adequacy theory is true, but not conversely. However, there is problem, because scientific laws are conditionals (implications). This suggests that adequacy is too strong conditions, because the consequence of an implication has a wider scope than its antecedent. Thus, laws should have the form of equivalence. The paper shows how model-theoretic characterization of theories allows to recognize truth and adequacy, consistently with Petrażycki’s claims.