 is a phrase which, like many others e.g., ‘vegetable’ one understands well enough in the ordinary way. But one can have difficulties when speaking to greengrocers or microbiologists or when playing ‘twenty questions’. Are rhubarb and tomatoes vegetables or fruits? Is coal vegetable or mineral? What about coal gas, marrow, fossilised trees, streptococci, viruses? Has the lettuce I ate at lunch yet become animal? … The same sort of difficulty arises about question c) above [Is there a systematic method by which I can answer such-and-such questions?]. An ordinary sort of acquaintance with the meaning of the phrase ‘systematic method’ won’t do, because one has got to be able to say quite clearly about any kind of method that might be proposed whether it is allowable or not. (Turing in Copeland 2004b: 590)