I wanted to check, as far as possible, whether this phrase in James Blish's Doctor Mirabilis was a misprint for "...Platonic world..." However, it is "...Platonic word..." in both of the editions that are in my possession.
The references are:
-James Blish, Doctor Mirabilis (New York, 1971), p. 37.
-James Blish, After Such Knowledge (London, 1991), p. 49.
The phrase is in Chapter II, "Northover."
However, I think that "world" would have fitted the context better. See here.