The Labour candidates each, in a way, represent a different analysis of the party's defeat.
For Liz Kendall, the party lost because of a lack of pragmatism and for putting out a manifesto that was too left of field. This is exactly the narrative which the Murdoch press is fond of, as you will recall they spun Miliband - a leader only slightly left of centre - as some kind of rabid Trotskyist pie-in-the-sky lunatic.
I have no idea what Yvette Cooper or Andy Burnham think about anything, except that they want to be the Labour leader. Burnham keeps insisting that the party's manifesto was great, so by elimination seems to think they lost because of external factors (which probably means a lack of media allies).
Corbyn, on the other hand, thinks that the party lost because of a lack of clarity. It wasn't clear how the party would govern without an SNP deal, or what that deal would be, it wasn't clear what their policy on Trident was, it wasn't clear how far they bought into austerity or didn't. About the only thing Ed Miliband was clear on was that he didn't want an EU referendum - which is, unfortunately for him, a fairly unpopular idea.
Of these three analyses, Corbyn is probably closest to the truth. A huge number of votes were cast to anti-austerity parties, and Labour would be in a much stronger position now if it could count on them. I also think he is right to suggest that Labour is better off putting forward its own programme of ideas rather than spitting out yet another vacuous generation of Tory-lite.
Of course, it's easy to talk about having principles generally, but it's harder to find the right ones. Whether Corbyn's principles are right ones really remains to be seen.