Line-By-Line Interpretation

Heaven and earth are enduring. The reason why heaven and earth can be enduring is that they do not give themselves life.

Heaven and earth are eternal and do not die or end. Because all that lives must die, heaven and earth do not die, and are thus enduring. Heaven and earth embody all, instead of taking on one specific individuality or life, they encompass everything, and they are everlasting.

Hence they are able to be long-lived.

Heaven and earth do not give themselves life, which is why they are able to be long-lived. They are not focused on the self, so they do not give life to themselves. They are everything, so they are long-lived.

Therefore the sage puts his person last and comes first,

The sage does not focus on his own life, his individual self. Instead, he puts it last by focusing more on the whole, and being more like heaven and earth. Because he does not focus on his own life and self, he is able to benefit and have a good life.

Treats it as extraneous to himself and it is preserved.

The sage separates himself from the way as a whole, and in this way he can preserve his own life. The sage does not think in terms of the mortal self, but eternal heaven and earth. In this way he preserves himself.

Is it not because he is without thought of self that he is able to accomplish his private ends?

The sage takes himself out of his own situation and because of this he benefits and is able to live a fulfilling life and accomplish things.