The "proof" of the title is plural. The brilliantly crafted play demonstrates almost limitless numbers; there is the mathematical proof which proves Catherine’s ability and the right to be her father’s daughter; there is proof of sibling affection, of passion inspired by discovery and of the instant connection which creates love.

Proof shows human interaction in its barest form, symbolised by the sagging chairs and newspaper piles on Robert’s old porch. Marie Ellis as Catherine drinks bad champagne from the bottle – she has jangling emotions, but no glasses.

It is a fantastic performance of subtle nuance and fierce drama, matched by the impossibly nerdy Hal of Robert Purchese who proves that age, love and beer can soften the twitchiest of geeks. Bridgett Ane Lawrence as cool Claire showed a touching fragility under her tightly controlled dress and manner – rumpled hair and looser clothing let rip jealousy and spite.

Dad – Bill Griffiths – has some of the best lines and much of the comedy. He is, after all, dead. Alive, he is a violent mixture of laid back indolence and guilty anger. Simple, effective set and superb direction from Claire Lewis combined to prove an evening of enthralling theatre.