As someone who is 3 years into writing a PhD thesis (in political science), what I write today is vastly different from what I wrote 3 years ago. This is because I have performed (written up my work following research - ala, gone out in the field)... Received feedback (I've been through 3 Annual Reviews, where my work has been absolutely torn to bits - ala posting on this forum for comments and criticism from other authorities on the subject)... and I have revised my work to the point that the style, substance, and even the very broad methodology which underpins my whole thesis has been completely overhauled (ala - becoming a PUA!).

It takes time, years and years, hard work, graft, and on occasions, really putting a lot of time in only to see your efforts written off, and roundly criticised from every possible angle!

But I have never responded negatively to criticism from my tutors, I have gobbled up every bit of advice they have given me, and I have used that to hone my work. I am in the middle of re-re-rewriting three of my draft chapters (I'm not even 30% done with this thesis yet!), but my approach today is so much more thorough, less superficial, and is based on much stronger principles and methods than it was three years ago.

And that is only because I have repeated that process: Performance. Feedback. Revision.