A nymph lies dreaming, her white body cushioned by green and brown grasses and sheltered by fanlike trees, under a sky of blue and grey. Two hairy-legged satyrs, creatures of lust and animal instinct in classical myth, have crept up on her. One of them grins as the other lifts a bit of white cloth to reveal her nudity.

I mention Nicolas Poussin's painting Nymph with Satyrs not just because I am writing this on Valentine's Day but because this painting recently changed my life. Maybe that's an exaggeration – but it changed my mind.

Poussin, who was born in 1594 and died in 1665, is unquestionably one of the giants of art. His paintings, with their classical and religious narrative ambition, have fascinated everyone from spies – Anthony Blunt was an authority on his work – to abstract artists such as Cy Twombly, whose affinity for Poussin will be explored in an exhibition in Dulwich later this year. But for a long time he left me cold. I knew it was my problem, not his. What I needed – it turned out, base creature that I am, is this painting. It finally encouraged me to see the passion of Poussin – and set me free to wander among the monuments of his profound art.