(The criticism is quoted by Gray: "If all scientific knowledge is tentative, and if social theories are identified as utopian on the basis of scientific knowledge, then all identification of social theory as utopian must likewise be
The critic claims here that KP employs "scientific knowledge" to identify utopian social theories; that KP asserts all scientific knowledge is tentative; and that KP's own philosophy demands that the scientist seeks to falsify the predictions of scientific theory, so there should - according to KP - be a test to "see" if the utopian social revolutionary social theiory is in fact utopian by, as it were, running the experiment, enacting the theory; so KP is being self-inconsistent by simply dismissing utopian social theories as non-scientific.)
I agree with Gray that the critic misunderstands Popper,and certainly I would not bother to reply to it. But that is the (weary) job of academics.
However, JG’s closing remarks in the essay have been of real interest to me. Here Gray identifies a link between Mill and KP, except that Mill’s liberalism and pluralism relies on a "naturalistic conception fo man", whereas KP’s rests on an anti-foundationalist epistemology, which Gray describes as "an ideally appropriate metaphysical perspective".