The whole and the ‘wholes’ must not be confused with sets. Sets are closed, and everything which is closed is artificially closed. Sets are always sets of parts. But a whole is not closed, it is open; and it has no parts except in a very special sense, since it cannot be divided without changing qualitatively at each stage of the division. ‘The real whole might well be, we conceive, an indivisible continuity.’ The whole is not a closed set, but on the contrary that by virtue of which the set is never absolutely closed, never completely sheltered, that which keeps it open somewhere as if by the finest thread which attaches it to the rest of the universe.
— Gilles Deleuze