possibility

Possibility is the condition of being that may occur or be true. In philosophy, it is a central notion of modality, the study of what could be the case in contrast to what must be the case.

Philosophical usage distinguishes several kinds of possibility: logical possibility (no internal contradictions), physical possibility (compatible with the laws of nature), metaphysical possibility (not ruled out by the fundamental nature of things), and epistemic possibility (not contradicted by what is known). These distinctions help clarify statements such as whether a given scenario could exist, occur, or be true under different restrictions or knowledge.

Examples illustrate the contrasts: it is logically possible that there exists a unicorn, because no contradiction forces its impossibility; it is physically possible to travel to the Moon with sufficient technology and resources; it is epistemically possible that it will rain tomorrow, given current information.

In formal logic, possibility is represented by the modal operator diamond (◇), with necessity by a square (□). Possible-world semantics define a statement as possible if it is true in at least one possible world. The de re versus de dicto distinction concerns whether the possibility is about a thing itself (de re) or about the truth of a statement (de dicto).

Applications extend beyond philosophy to probability, uncertainty modeling, and artificial intelligence. Possibility theory and related frameworks address how information about what could be true is structured and used when exact probabilities are unavailable. Overall, possibility provides a foundational language for discussing constraints, imagination, and knowledge about what could be.