A philosopher shows that physicists use different definitions of the concept, depending on their own particular fields of interest.
Phenomena such as black holes belong to a realm that is inaccessible to observation and experiment.
Work based on the assumption that black holes exist therefore involves a level of speculation that is unusual even for the field of theoretical physics.
The physical perspective on black holes is itself inextricably bound up with philosophical issues relating to ontological, metaphysical and methodological considerations.
For astrophysicist, "a black hole is the ultimate prison: once you check in, you can never get out."
On the other hand, theoretical physicist regards it as "conceptually problematical to think of black holes as objects in space, things that can move and be pushed around."