This paper discusses the role of endoreversible thermodynamics as a part of the first of three phases of applying thermodynamics to the process of system design: the phase of concept generation. In this first phase, we consider only the input material and energy and output material and energy. Under this limited situation, we can derive an inequality to judge the feasibility of the system. Such a method has a clear meaning at the very early stage in which we create various conceptual approaches and examine them. In endoreversible thermodynamics, this method is linked to optimization. However, to apply optimization, we should advance to the higher phases of the system design.