A challenge for the requirements engineering (RE) field is to ensure simplicity of requirements. Often the real needs are hidden beneath contextual knowledge, assumptions, inconsistencies, and even omissions. Our goal is to state requirements simply, to promote simpler designs and thus simpler systems. Similarly, we seek to apply the complementarity principle as we write requirements. Every concept has a complement or opposite: cause/effect, input/output, start/stop, etc. In RE, we study the system and the human. We seek ways to perform RE such that we allocate the tasks that require a human being's unique abilities to the humans and automate the other system tasks. Here is where the complementarity principle can come into play and drive our work.