In chapter IV of Part Two of his Being and Nothingness J.P. Sartre gives a, surprisingly, clear description of some themes related to the question of the being of the possible – or the being of possibilities. The notion of the possible has, of course, been programmatically criticized by Deleuze who – following Bergson’s Creative Evolution – sees in the ‘possible’ a false problem; one that (i) reduces to an ‘either..or’ dichotomy something that must be seen of admitting of degrees (‘more or less’) and/or (ii) confuses the ‘more’ with the ‘less by thinking that in reverses hierarchies (‘the real is ‘more real’ than the possible etc.; ‘the real’ = the possible + existence). Deleuze proposes an alternative dual notion – that of the actual and the virtual, where the virtual is fully real but only actualized. In effect, Sartre’s passage could be understood as a beautiful example of looking for a certain mode of existence in the wrong place; he tries to retrieve something from ‘the possible’ that, perhaps, was never part of it. Yet, somewhat (!) similar to Deleuze he holds that a ground (e.g. of the possible) does not resemble that what it grounds. For Sartre, therefore, the possible is grounded in what it is not – a ‘lack’, as he would call it. Unlike Deleuze, this lack belongs wholly to the for-itself, in so far as it is consciousness that always already ‘makes itself’.
“With the possible [..] there is the greatest difficulty in understanding its being, for it is given as prior to the being of which it is the pure possibility; and yet qua possible, at least, it necessarily must have being. [..] Since Leibniz the term “possible” is usually applied to an event which it is not engaged in an existing causal series such that the event can be surely determined and which involves no contradiction either with itself or with the system under consideration. Thus defined the possible is possible only with regard to knowledge since we are not in a position either to affirm or to deny the possible confronted. Hence we may take two attitudes in the face of the possible: We can consider, as Spinoza did, that possibilities exist only in connection with our ignorance and that they disappear when our ignorance disappears. In this case the possible is only a subjective state on the road to perfect knowledge; it has only the reality of a psychic mode; as confused or curtailed thought it has a concrete being but not as a property of the world. But it is also permissible, as Leibniz does, to make of the infinity of the possibles objects of thought for the divine understanding and so confer on them a mode of absolute reality [..] This [e.g.] means only that there exists by virtue of the thought of the divine understanding another system of co-possibles such that Adam figures there as having not eaten the fruit of the Tree of Knowledge. But is this conception so different from that of Spinoza? Actually the reality of the possible is uniquely that of the divine thought! This means that it has being as thought which has not been realized. [..] [Now] to give possibles a tendency toward being means either that the possible is already in full being and that it has the same type of being [..] or else that the possible in the bosom of the divine understanding is already an idea-force and that the maximum of idea-forces organized in a system automatically releases the divine will. But in the latter case we do not get out of the subjective. If then we define possible as non-contradictory, it can have being only as the thought of a being prior to the real world or prior to the pure consciousness of the world such as it is. In either case the possible loses its nature as possible and is reabsorbed in the subjective being of representation. But this represented-being of the possible can not account for its nature; on the contrary it destroys its nature [..] To comprehend possibility qua possibility or to be its own possible is one and the same necessity for the being such that in its being, its being is in question. But to be its own possibility – that is, to be defined by it – is precisely to be defined by that part of itself which it is not, is to be defined as an escape-from-itself towards -. (p. 96-97).