Tom Stoppard has written one of the great political plays in the English language, and like all great political plays, it resonates with humanity. It has a moving, throat-catching intensity that reminds me of Arthur Miller summing up Tennessee Williams’s plays as “the politics of the soul”. Stoppard has always been a hard-line humanist, and this play shows him at his combative and tolerant best. Proust thought that art was the true last judgment, which is pushing it a bit, but Stoppard shows that great theatre is the nearest you can get to it on this bitch of an earth.

The play is set in Cambridge and Prague from 1968 to 1990. Max (Brian Cox) is a philosophy don and an old-fashioned communist who didn’t leave the party in 1956, when Khruschev crushed the Hungarian revolution, or in 1968, when Brezhnev crushed the Prague Spring. Max despises the idea of a “reformed” communist; he believes in basic premises and cannot see that dictatorships will lead only to some fraudulent and murderous utopia.

Jan (Rufus Sewell) is his pupil, an émigré Czech and a hard-line idealist who returns to Prague in 1968 to defend the socialism of his dreams. He is also a rock’n’roll junkie devoted to a Czech band called Plastic People of the Universe. He goes home partly to save rock’n’roll. What he doesn’t realise is that the commissars who rule his country regard the PPU’s work as “socially negative music”. Negative is bad news. You can be arrested for being negative. Positive is good news: a code word for following party directives. Soviet communism was an example of illogical positivism. Jan will have to learn the hard way.

The play has two great themes. One is the cost of integrity. Max can’t accept the failure of his ideals: it would be defeatism that would turn disaster into moral victory. Jan, back in Prague and still optimistic, thinks that signing a protest would be moral exhibitionism. The other theme is the nature of freedom. Max’s wife, Eleanor (Sinead Cusack), is a classicist, and through her you hear the echoes of ancient mythologies: the great god Pan, for example, whose music is an instrument of liberation and pleasure. He is the ancestor of the popular music of the late 20th century. Rock’n’roll and its siblings were a shout for freedom: freedom from convention and an oppressive bourgeois lifestyle. In communist Prague, you had no lifestyle. Life was all you could hope for. Here, rock’n’roll meant defiance, a cry for the freedom to be yourself. This play could have been called The Invention of Freedom.

Stoppard doesn’t take sides. He once said that writing dialogue was the only way he could contradict himself, and here he demonstrates the power of the dramatist who can enter the mind of his characters and conduct that inner dialogue of mind and soul through which we try to understand the world. This is not a play reserved for classicists, philosophers or political scientists. It echoes with Stoppard’s humour: sad, acid, elegant and subversive. Under and through its arguments beats the steady pulse of humanity, of the accountability people need to have for themselves and each other. One thing Eleanor learns from the classics is that there’s a spirit within man that opposes him being just machinery. Max, Marxist to the core, thinks that love is only economics and physiology, but Eleanor knows better. She knows that you can have subjective experience in an objective world. If there’s no soul, it would have to be invented. This is what drives the harrowing scene between Max and Eleanor in the second half — the emotional peak of the play, where Cox and Cusack are at the peak of their unforgettably human performance.

Trevor Nunn’s production is a masterpiece of lucidity, intelligence and feeling. The showman of theatrical spectacle has done some of his best work on small stages, where intimacy can best reveal the soul about which Max is so robustly sceptical, and of which the great god Pan is an unacknowledged guardian. So were Pink Floyd when they defied the logical positivists and sang: “I don’t care if the sun don’t shine.”