Who get real happiness from their activity?

On their own feet they came, or on shipboard,    
Camel-back, horse-back, ass-back, mule-back,    
Old civilisations put to the sword.    
Then they and their wisdom went to rack;    
No handiwork of Callimachus,    
Who handled marble as if it were bronze,    
Made draperies that seemed to rise    
When sea-wind swept the corner stands:    
His long lamp-chimney shaped like the stem    
Of a slender palm, stood but a day;    
All things fall and are built again,    
And those that build them again are gay.    
— WB Yeats