Various web sites list this quote, but without a pin cite; the Amazon Look inside seems to confirm it, but I couldn't view the page (p. 171 given). QuoteFancy.com gives a slightly longer quote: “A metaphor is not an ornament. It is an organ of perception. Through metaphors, we see the world as one thing or another.”
ThoughCo.com offers a few sources for this quote, but neither are from Postman:
Gerard Genette on the Recovery of the Vision
Thus metaphor is not an ornament, but the necessary instrument for a recovery, through style, of the vision of essences, because it is the stylistic equivalent of the psychological experience of involuntary memory, which alone, by bringing together two sensations separated in time, is able to release their common essence through the miracle of an analogy — though metaphor has an added advantage over reminiscence, in that the latter is a fleeting contemplation of eternity, while the former enjoys the permanence of the work of art.
(Gerard Genette, Figures of Literary Discourse, Columbia University Press, 1981)
I.A. Richards on the Omnipresent Principle of Language
Throughout the history of Rhetoric, metaphor has been treated as a sort of happy extra trick with words, an opportunity to exploit the accidents of their versatility, something in place occasionally but requiring unusual skill and caution. In brief, a grace or ornament or added power of language, not its constitutive form. . . .
That metaphor is the omnipresent principle of language can be shown by mere observation. We cannot get through three sentences of ordinary fluid discourse without it.
(I.A. Richards, The Philosophy of Language, 1936)
From the Rattle page:
"Much like George Lakoff and Ludwig Wittgenstein, Postman maintained that words (and words, in truth, are metaphors) are as much the driver of reality as they are the vehicle. . . . For Postman, the study of metaphor was unending and metaphors were as crucial as they were omnipresent; they served to give form to and dictate experience. Is America the great melting pot, or is it an experiment in unity through diversity? What metaphors are embedded in television commercials . . . Put simply, Postman (like his teacher and hero, Marshall McLuhan), maintained that the medium through which information is conveyed directly colors meaning and our sense of the world . . . We are, essentially, what we see, hear, and read. Postman might go so far as to opine that we are the metaphors we use."
This makes me wonder about metaphor shading into myth...