The compossibility is a philosophical concept defined by Leibniz. It has a logical sphere more restricted than that of the logical possibility. To exist it is not enough that something is possible, it is necessary that this thing is compossible with others which constitute the real-world. That indicates the whole of continuity made up. It is when of the series the ordinary ones converge, small regular points which derive from two singularities and when their values coincide.

" the concept leibnizienne of compossible retains the fundamental idea whereby the possible one is determined by the possible." Roberto Miguelez (rules of the interaction, Tests in sociological philosophy p.299, presses of the Laval University). The term is also employed by David Lewis.

At Leibniz, the compossible one is associated with the possible idea of world. Our world with the characteristic to be Brave New World possible, but it is only one among others. In these other worlds, the things could have been radically different from what they are in our world. Certain truths necessary in our world, like the natural laws, could not exist in other world. This is why it is necessary to distinguish the possibility in all the worlds, and the possibility inside a world, which is the compossibility.

If one takes the whole of the possible worlds, then one can say that all that is not contradictory by oneself is possible. A thing which does not exist in our world, or which cannot exist, can exist very well in another. On the other hand, inside our world, all is not possible. Because certain things, in fact exist, and of the things which would contradict those which exist could not come to the existence. From where the term of compossible: a thing is compossible which is possible taking into account all the things which are in the world. " today would be said; compatible". That explains the " cum" (" avec") of " COM-possible". A thing is compossible if it remains possible, not absolutely, but when it is with the existing things.