The Birth of Tragedy is Nietzsche's first book and his attempt to explain where Greek tragedy came from and why it mattered. He sets aside the calm, cheerful Greece of textbook admiration and looks for the darker need that drove the Greeks to create art at all. His tool is a single contrast, drawn from two of their gods, that he believes runs through the whole history of art.
Apollo is the god of light, form, and measured limit. The Apollonian impulse is the power that shapes clear images, the dream-world in which every figure has its outline and its beauty. Dionysus is the god of wine and ecstasy. The Dionysian impulse breaks down the boundary of the individual, drowns the self in music and intoxication, and reunites a person with nature and with other people. One impulse builds the separate form; the other tears it down.
These two pull against each other, yet Nietzsche argues that their forced union produced the highest Greek art. Tragedy grew out of the satyr chorus and its Dionysian music, the formless surge of feeling, which the Apollonian then gave a face: the suffering hero on the stage is the one god Dionysus appearing in many masks, read back to the audience through Apollonian image and speech. Music supplies the depth; the visible drama supplies the shape.
Behind all this stands a hard view of life. The Greeks felt the horror of existence keenly, as the captured Silenus tells King Midas that the best of all is not to be born and the next best is to die soon. They did not deny this. They built the bright Olympian gods and the beauty of art over the abyss so that life could be borne and even loved. For Nietzsche this is the deepest function of art: it does not preach or improve us, it makes a terrible existence endurable by turning it into something worth beholding.
The book then tells a story of loss and hope. Tragedy died, Nietzsche says, by a kind of suicide, when Euripides put a new spirit on the stage and Socrates taught that virtue is knowledge and that whatever cannot be made fully rational is worthless. This theoretical optimism drove out the Dionysian and dissolved myth. Yet Nietzsche ends looking forward, hearing in the spirit of music, above all in Wagner, the promise of a rebirth of tragedy and of a culture once more able to face suffering without flinching.