Think constantly of all the sorts of men, of various professions and of all the nations on earth, who have died: and so bring your thought down to Philistion, Phoebus, and Origanion. Pass now to the other classes of men. We too are bound to change our abode to that other world, where there are so many skilled orators, so many distinguished philosophers - Heraclitus, Pythagoras, Socrates - so many heroes of old, so many later commanders and kings.
Add Eudoxus, Hipparchus, Archimedes; add other men of penetrating intellect, men of great vision, men dedicated to their work; add rogues, bigots, and even satirists of this transient mortal life, like Menippus and his kind. Reflect of all of these that they are long dead and buried. So is this anything terrible for them - or indeed for men whose very names are lost? In this world there is only one thing of value, to live out your life in truth and justice, tolerant of those who are neither true nor just.
Upon further reflection, you will notice everyone passes on to death and oblivion. Pillars of men and nations disappear and succumb to The Agent of Change. What's left? What's the purpose? What is it all about? It is about virtue; justice. Finding unalterable truth - digging and searching until you find that foundation of bedrock that won't move under your feet. You will find a life in pursuit of attaining virtue (wisdom, justice, courage, temperance) is the only thing of value. Pursuit of fame, fortune, notoriety, pleasure / avoiding pain ... all futile and useless.
(see also Citadel p. 48, 164, 177-178, 228, 242, 276, 286)