Imagine two people, one with depression and one with cancer. Both patients are clearly able to make rational judgments about their treatment. They both recognise they are ill and that there are consequences to not receiving treatment. In the case of the former, the fact the patient has capacity to make clear and rational decisions has no relevance; and even if fully capable, their unwillingness to consent can be overridden. In the case of the latter, their capacity is central, and proceeding to treatment without their consent would be an assault.