The phrase was coined by Prof. Eugen Rosenstock-Huessy, and (apparently) offered as the motto of his life's work. Its philosphical implications are examined in "Farewell to Descartes" in I am an impure thinker (Argo Books, 1969).
I quote Prof. C.A. Coulson [1][2][3] from his book Science & Christian Belief, published in 1960:Descartes could use the argument, ``cogito, ergo sum'' as the basis of his philosophy. But it seems to me now that he only said one half of what there is to say. [...] Descartes describes man-the-spectator, man-the-observer, with the power to use his thinking to make a descriptive pattern of the universe in which he lives. But man is more: he is all the time responding to his environment: he forsees, he decides, he acts, he controls. We are not truly human unless we are both actor and spectator. This is clear enough if we recall the terms in which we describe a man who always seems to be separate from the things that go on in the world; watching them carefully, sometimes studying them, but quite unperturbed and unaffected by them, unsympathetic, apparently neither rejoicing nor being sad about events. We say: what an inhuman sort of man he is! [...] Dr. F.H. Heinemann has put this very neatly by varying Descartes' argument to read: ``Respondeo, ergo sum''.
[...] Man and Nature have been bound together in an inescapable intimacy. No account of Nature can be given other than in personal terms. [...] To realise this personal character of everything that we see and touch and handle is to undergo an internal revolution. For our attitude to ``things'' when they are regarded simply as things, is vastly different from our attitude to persons. [...] I believe that the act of reflection, by showing us the personal character of reality, profoundly affects our relationship to it.
[...] To recognise the personal character of reality is to be changed. This means that [...] it was not really sufficient to takes Descartes' ``cogito, ergo sum'' and add to it Heinemann's ``respondeo, ergo sum''. It was certainly correct to make the addition but we must go further [...]. Professor Rosenstock-Huessy has urged that this extra keynote can be put in the form ``respondeo, etsi mutabor'' -- I respond, although I shall be changed. To face up to the totality of what we experience in our environment -- both natural objects and human beings -- in the act of reflection, is to risk the almost certain chance of being changed.
I find related thoughts echoed by:
Stephen Talbott edits the electronic newsletter NetFuture.