Banksy's new mural in Marseille is not the first image he has connected to the history of ideas. From Plato to Foucault, a Banksy expert reveals the philosophy behind these popular artworks.

Which is the real you, the person you are now or the one you are capable of becoming? It's a heady question, to be sure, and not one you would expect to be confronted with while strolling down a street in Marseille in the waning days of May. Yet it's precisely the existential dilemma that Banksy, who once asserted "being yourself is overrated" – has surreptitiously installed in a cloistered stretch of the quiet Rue Félix Fregier, the site of a new work – the latest installment in the elusive artist's decades-long career as a provocative philosophical prankster.

For more than 30 years, Banksy has spiked many of his most iconic works – from his girl reaching hopelessly for a heart-shaped balloon to his masked rioter hurling a bouquet of flowers – with barbed allusions to Old Masters, from Michelangelo to Monet, Vermeer to Van Gogh. But there's more. Beneath his stealthy stencils lies a deep and deliberate engagement with the history of ideas as well, from classical Stoicism to postmodern deconstructionism.

On 29 May, Banksy posted on Instagram a photo of his first new piece in more than five months, piquing the internet's interest by withholding its precise location. Discovered shortly thereafter in the major port city in southern France, Marseille, the mural is, at first glance, deceptively simple: a tall silhouette of a lighthouse spray-painted on to a blank beige urban wall; a rusting street bollard positioned nearby; and a painted shadow stretching across the pavement, joining the real-world object to its augmented, if two-dimensional, echo. Stencilled across the black lighthouse are the words: "I want to be what you saw in me."

Anyone keen to find a source for the ideas that inform Banksy's new work needs merely to flip open any history of philosophy to Plato's seminal allegory of the cave (from the Fourth-Century BC treatise The Republic), then flip the ancient metaphor on its head. In Plato's parable, prisoners chained inside a cave mistake shadows on the wall for reality, unaware of the truer forms that cast them outside. But here, Banksy, being Banksy, baits us by switching the set-up, reversing the relationship between essence and shadow. In Banksy's mural, the drab bollard casts not a diminished imitation of itself, but something far grander – a lighthouse, a symbol of illumination and guidance. Here, it's the silhouette, not reality, that's true.

Banksy's inversion urges us to ask where reality really resides: in what is, or in what might be? His poignant phrase – "I want to be what you saw in me" – is alluringly elastic. Is this the bollard dreaming of being more than it appears? Or the shadow wishing to become light? Or is it all of us –........

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