When is a student ready to finish his/her PhD?

I’ve made it a hobby for myself to ask this question to professors that I meet at conferences in my field. The answers that I have collected in these conversations are manifestations of an astonishing variety of underlying research philosophies and ideologies. Here’s a list of answers I have received so far, the labels in brackets are mine, they might be misleading, deceptive or misrepresent the original intent of the answer given.

• When he is offered a position in industry or academia that assumes a PhD (the american view)
• When he has convinced his corresponding research (sub-)community that the work he has been doing is worthy of a PhD (the sociologist’s / psychologist’s view)
• When he has expanded the state of knowledge by a significant amount / When he added new knowledge to the existing body of knowledge about the world (the epistemological view)
• When he has built something truly new, interesting, elegant and/or complex (the engineer’s view)
• When he has reached his personal intellectual maximum i.e. the maximum intellectual capacity that he is capable of acquiring (the subjective view)
• When he is able to explain the results of his work in one sentence (the communication view)
• When he has published n papers (the bureaucrat’s view)

I am amazed that there is little repetition in the answers that I get. What is your answer? Add it to the comments.