I did some digging and the principles are elucidated by vitalik on his blog.[0] The principles are based on the fact that proof of work is antisocial, impractical or unrealistic. The 6 times this was discussed last year[1] in the ethereum devs meeting on zoom might also be of-interest.

[0]: https://vitalik.ca/general/2016/12/29/pos_design.html [1]: https://github.com/ethereum/pm/issues/361