In large-scale, real-time systems, the software design process is still highly dependent on the skills of the developers. To enable the efficient, speedy design of reliable software products, we require a means of conveying design decisions from experts to other engineers. Our approach involves the development of the "Causality of Problem-Issue-Solution" (CAPIS) model which can be used to represent experts? thought processes. The CAPIS model divides a thought process that includes complexity and diversity into Problems, Issues, and Solutions (PIS), and then describes conceptual models based on the knowledge hierarchy of Data, Information, Knowledge, and Wisdom. The PIS ontology is used to describe items in the conceptual models. This paper describes a software design method that is based on this CAPIS model. As an example, we consider the thought processes involved in building reliability into a UML class diagram.