. A problem-solving method describes a reasoning process that efficiently achieves a goal by applying domain knowledge. However, a problem-solving method cannot directly be applied because of the existence of a gap between, on the one hand, a problem-solving method and the domain knowledge it uses, and, on the other hand, a problem-solving method and the goal that it is supposed to achieve. In this paper, we distinguish two types of assumptions based on an architecture of problem-solving methods, that are able to bridge the gap: one type of assumption is used to strengthen a problem-solving method, and the other to weaken the goal to be achieved. We also show how the effect of one assumption type can be substituted by the effect of the other type, and refer to this as "the law of conservation of assumptions". 1 Introduction The notion of problem-solving method (PSM) is present in many current knowledge engineering frameworks such as Generic Tasks , Role-Limiting Methods , KADS ..