Cats don't just purr when they are contented. Therefore, you need to find an answer to the question which covers all circumstances under which domestic as purr. Dr Desmond Morris, the celebrated zoologist and animal behaviourist came up with the right answer in 1986. I'm a bit surprised that is not more widely cited so I will mention it here. In fact, I will quote him verbatim if I may: "A more precise explanation, which fits all cases, is that purring signals a friendly social mood, and it can be given as a signal to, say, a vet from an injured cat indicating the need for friendship, or as a signal to an owner, saying thank you for friendship given". I don't think I can explain it any better than that. The point that he is stressing is that domestic cats purr when they are in a veterinarian's clinic, on the consulting table where they are clearly nervous and stressed. And sometimes they purr when they are about to be euthanised by a veterinarian. Clearly