Le Seize Septembre (translation: "September Sixteen") depicts a lone tree at dusk. It looks like an ordinary scene.
Yet if you look close you will notice a crescent moon superimposed on the tree. Perhaps Magritte is reminding us to keep in mind that a painting is not reality but rather an image that the artist sees in his mind's eye. If you know the moon is behind the tree, with your mind's eye you can see the moon.