There are conceptual distinctions that have been repeatedly introduced into philosophical reflection and that have been correspondingly disappeared into nothingness without leaving a visible trace of their presence. The systematic constant disappearance of these distinctions after short periods of presence means perhaps that it is a question of false distinctions. But the fact that they keep on appearing again in different philosophical contexts, theories and systems perhaps sends us back to the presence of a theoretical impasse or of a conceptual knot which is not solved yet.