Was ist Thanatos?

From Nikos Kazantzakis's Zorba the Greek:

"And why the devil d'you have to go down to the sea to make calculations? Pardon me, boss, for asking this question, but I don't understand. When I have to wrestle with figures, I feel like I'd like to stuff myself into a hole in the ground, so I can't see anything. If I raise my eyes and see the sea, or a tree, or a woman--even if she's an old 'un--damme if all the sums and figures don't go to blazes. They grow wings and I have to chase 'em..." 

"But that's your fault, Zorba," I said to tease him. "You don't concentrate." 

"Maybe you're right, boss. It all depends on the way you look at it. There are cases even wise old Solomon... Look, one day I had gone to a little village. An old grandfather of ninety was busy planting an almond tree. 'What, grandad!' I exclaimed. 'Planting an almond tree?' And he, bent as he was, turned round and said: 'My son, I carry on as if I should never die.' I replied: 'And I carry on as if I was going to die any minute.' Which of us was right, boss?"