What is Lucretius symmetry argument?

Lucretius, answered from the texts and cited to the page.

Lucretius answers

The argument is this: the time before we were born has been nothing to us, and nature holds that time up as a mirror of the time to come.1 Look back over all the ages of everlasting time that passed before our birth, and ask whether any of it troubles you. It does not. Then the ages after our death are the same condition, and there is nothing in it that looks terrible or gloomy.

It is a calmer rest than any sleep.1 The reasoning beneath the mirror runs through the mortal soul. Mind and soul are born with the body, fall sick with it, and are scattered when it is.2 So when the parting of body and soul has come, nothing at all can happen to us, who will then be no more. We felt no ill when the Poeni came from every side to the shock of battle and the whole world shuddered under the war.

In the same way, when we are no more, nothing can stir our feeling, not though earth be mingled with sea and sea with sky.3 The ground of it is plain. If grief and pain are to befall a man, he must himself exist at that time for the ill to reach him. Death forestalls this and prevents the being of the one on whom misfortune might crowd, so we have naught to fear in death.3

The man who pities his own corpse has left some part of himself standing by to feel it, and that is the error. I add a second turn. Even if time should gather our substance together again after death and restore the light of life, it would not concern us, once the memory of our former selves was snapped in two. We do not now care for the selves we once were, nor does any pain for them touch us.3

And the old man who weeps over his going hears nature's rebuke: life is granted to none for freehold, to all on lease. One generation yields to the next, and those who come will follow him when they have had their fill.4

Sources

  1. Look back again to see how the past ages of everlasting time, before we are born, have been as naught to us. These then nature holds up to us as a mirror of the time that is to come, when we are dead and gone.
    On the Nature of Things, pp. 138–139
  2. Death, then, is naught to us, nor does it concern us a whit, inasmuch as the nature of the mind is but a mortal possession.
    On the Nature of Things, pp. 133–134
  3. For, if by chance there is to be grief and pain for a man, he must needs himself too exist at that time, that ill may befall him. Since death forestalls this, and prevents the being of him, on whom these misfortunes might crowd, we may know that we have naught to fear in death
    On the Nature of Things, pp. 134–135
  4. life is granted to none for freehold, to all on lease.
    On the Nature of Things, pp. 138–139