Phædo or Phaedo (; , Phaidōn, ), also known to ancient readers as On The Soul, is one of the best-known dialogues of Plato's middle period, along with the Republic and the Symposium. The philosophical subject of the dialogue is the immortality of the soul. It is set in the last hours prior to the death of Socrates, and is Plato's fourth and last dialogue to detail the philosopher's final days, following Euthyphro, Apology, and Crito.