天长地久。

Heaven is eternal and Earth everlasting.

天地所以能长且久者，一起不自生，

They can be eternal and everlasting because they do not exist for themselves.

故能长生。

And for this reason they can exist forever.

是以圣人后其身而身先，

Therefore the sage places himself in the background, but finds himself in the foreground.

外其身而身存。

He puts himself away, and yet he always remains.

非以其无私耶，

Is it not because he has no personal interests?

故能成其私。

This is the reason why he is able to accomplish his private ends.