 Two subsets "A" and "B" of a topological space "X" are said to be separated by neighbourhoods if there are neighbourhoods "U" of "A" and "V" of "B" that are disjoint.In particular "A" and "B" are necessarily disjoint.Two plain subsets "A" and "B" are said to be separated by a function if there exists a continuous function "f" from "X" into the unit interval [0,1] such that "f"("a") = 0 for all "a" in "A" and "f"("b") = 1 for all "b" in "B".