The phd student researches to proof/refute the thesis of his studies. Most studies are done to proof a thesis. To get the degree as a scientist you need to defend your thesis in front of a board of specialists and non-specialists.

I can remember the phd defense of my wife: Man, was I so tense I could have made diamonds from pieces of coal with my bare hands. 

Guido

Edit: Whoops ther answers are coming more quickly than I could type!