One of the big concepts Invisibility tackles is existence. Questions about the nature of existence fall under a branch of science known as ontology.
Scene 1 opens with the characters B, C and D debating the nature of A’s existence. A’s name is just ‘A’ – he does not have a ‘proper’ name, which puzzles the characters. He also does not have a job.
But A still exists – there has to be something else to A’s existence other than a name and a job.