The Tragedy of Thomas Hobbes, Wilton's, London <br>Imagine This, New London, London<br>Treasure Island, Theatre Royal, Haymarket, London

Of science, we expect cool reason, solid facts, firm conclusions; of theatre, passion, ambiguity, uncertain outcomes. In The Tragedy of Thomas Hobbes, the two worlds merge, so that science becomes the more flexible and creative art form, the stage's response to it, preening and grotesque.

The stern philosopher of the title, frustrated by the failure of his leaden pamphlets, rashly writes a staged dialogue in which he challenges his rival, the wealthy and devout scientist Robert Boyle. The result is one of the many splendid moments in Adriano Shaplin's often amusing play for the RSC, with all the farcical elements of unfortunate timing, humiliation and revelation.

Hobbes's audience are the questing scientists and amateurs who are the founders of the Royal Society: Boyle himself, neurologist Thomas Willis, astronomer John Wilkins, and the mathematician John Wallis – "WWW, easy to remember", as they are introduced to the newly crowned King Charles II, Arsher Ali's languid, Russell Brand-alike dilettante. For Cromwell is dead; the actors, Rotten and Black, can practise their dark arts again, and science, as long as it is flashy, like the king's wristwatch – "accurate to within two or three hours a day" – will get valuable royal patronage.

Boyle and his dextrous but malformed assistant, Robert Hooke, are grappling with the nothingness of air. As Hooke's observers freeze around the demonstrating table we see the shade of Joseph Wright of Derby's painting Experiment on a Bird in an Air Pump, the obscure shadows of ignorance broken by flashes of illumination.

Shaplin's play about the pursuit of knowledge and the ossification of experience is given the full RSC treatment at Wilton's, itself a miracle of preservation, all decked out with ladders, scaffolds and galleries for this production, and with electric violin and cello punctuating the action.

Amanda Hadingue as Boyle points up his unworldly otherness, with Jack Laskey wild and restless as Hooke, Stephen Boxer white with indignation as Hobbes, and James Garnon and Angus Wright poutingly outré as the thespians. It is a bold playwright who opens his second act with the Rotten protestation, "What is the meaning of this play? It's ranting shit!" Shaplin pulls it off.

Another show, another play within a play, this time "Masada", staged by the Warshowsky theatrical family, who are herded into the Warsaw ghetto after a blissful last summer's day at the beginning of Imagine This.

No one can fault the nobility of intention in Shuki Levy and David Goldsmith's musical. They head into deep waters at a time when many stick to the shallows of pop nostalgia and shows of the film of the book of the recipe. Opera, after all, thrives on its themes: persecution, heroism, love across the divide, faith, and the spiritual uplift of art. But Springtime for Hitler casts a long shadow across the Holocaust as seen from the musical stage, and there are moments in Imagine This when the laugh and the gasp come in the same breath. "You'll love it," says Daniel, played with distinction by Peter Polycarpou, to convince a Nazi commander that "Masada", the story of Jews persecuted by the Romans, will be appropriate for the ghetto audience. "It has singing, dancing, and all the Jews die in the end."

Sing and dance they do, raising fine voices proudly above the fear and loss, and taking the occasional side-turn into novelty numbers, plain daft staff-thumping and comic riffs. And this, in the end, is the problem. The deaths are too many, the memories too painful, the cruelty too unspeakable to be borne by this art form. "They say prepare to die," sings one victim, "and now I feel like dancing." I think not.

Tom Haines's original music for Treasure Island occasionally bubbles up into a brief chorus, but matters piratical off the coast of Somalia made this scramble for buried treasure look like a pedalo outing. Studiously avoiding the dot and carry-one gait traditionally accorded Long John Silver, Keith Allen lists 10 degrees to port, dragging a huge leather-clad false leg in his wake, setting about assailants with his crutch and threats rasped in a flat mockney drone. The hearty cast persevere with the aid of many ropes and barrels to navigate Ken Ludwig's faithful if hefty adaptation of Robert Louis Stevenson's classic tale. But for all the aerial choreography and gunfire, the default position is for characters to stand in a row and shout their lines at the audience. My seven-year-old companion took it all in and then pronounced his best bit to be Ben Gunn's yearning for cheese. It's rewarding enough, then, but not solid gold; more like finding a fiver in the pocket of an old coat.

'The Tragedy of Thomas Hobbes' (0844 800 1110), to 6 Dec; 'Imagine This' (0844 412 4654), booking until 28 Feb; 'Treasure Island' (020-7930 8800), to 28 Feb