In primary metaphor theory, a compound metaphor is a metaphor formed by unification of more primitive primary metaphors. In other words, while a primary metaphor relates two 'simple' concepts from distinct domains, in contrast, compound metaphors relate entire complex domains of experience. A celebrated example of a compound metaphor is theories are buildings, as evidenced by an example such as: Your theory lacks a solid foundation. Since both theories and arguments are relatively complex and rich in detail, they do not qualify as a primary target concept nor a primary source concept respectively.