I am very grateful to Dr Clifford for his letter. He is entirely correct to highlight the slippery slope. There are several slippery slopes here. Will the practice of euthanasia on the basis of suffering resulting from mental illness alone expand to less severe forms of mental illness, to earlier mental illness and to people without mental illness themselves but who experience substantial suffering as a result of mental illness in someone else (e.g. a family member)? The reason why I did not present the slippery slope argument initially was because the argument can, ironically, become a slippery slope itself, as skilled rhetoricians invoke all kinds of unlikely speculative scenarios with substantial emotional power, but limited practical relevance. Nonetheless, Dr Clifford's point is clearly right and I am especially pleased that he agrees with the central point of my commentary: we should not kill our patients.