From Wikipedia. Modality is an important concept for OSE, since OSE lives in the world of possibility - not actuality. An important distinction lies in identifying whether one is a possibilist or an actualist - similar to a realist vs idealist.
Modality concerns the concepts of possibility, actuality and necessity. In contemporary discourse, these concepts are often defined in terms of possible worlds. A possible world is a complete way how things could have been. The actual world is one possible world among others: things could have been different than they actually are. A proposition is possibly true if there is at least one possible world in which it is true; it is necessarily true if it is true in all possible worlds. Actualists and possibilists disagree on the ontological status of possible worlds. Actualists hold that reality is at its core actual and that possible worlds should be understood in terms of actual entities, for example, as fictions or as sets of sentences. Possibilists, on the other hand, assign to possible worlds the same fundamental ontological status as to the actual world. This is a form of modal realism, holding that reality has irreducibly modal features. Another important issue in this field concerns the distinction between contingent and necessary beings. Contingent beings are beings whose existence is possible but not necessary. Necessary beings, on the other hand, could not have failed to exist. It has been suggested that this distinction is the highest division of being.