Re: "It is not a tautology that some things (like people, and probably worms) feel, and that others (like stones, computers, and today's robots) don't."

No. But this is not what I said the tautology was. It is a tautology to say (eg) that "We all know what it feels like to feel." 

Re: "Meanwhile you keep missing the substantive point at issue: that feeling is something that can either be present or absent,"

Interesting point because it helps highlight the problem I have been getting at. If, as you claim, we all know 'what it feels like to feel', then presumably we would all know what it feels like not to feel? (E.g. it would make no sense to say we know what it feels like to be warm if we did not know what it feels like to be cool/cold - the word 'warm' would make no sense to us.)  Now, I have no idea what it is like not to feel (I don't mean in a localised sense like a local anaesthetic but in the global sense the word 'feel' seems to be intended in these discussions). In fact the only humans who, I imagine, 'know' what it like not to feel are dead humans - and perhaps those in a deep coma. And they cannot tell us - or make the required comparison. In other words, 'feel' in the sense the term is being used in the present context has no more meaning than the word 'warm' would in a world in which there is no cold.

DA