Student or Learner
Boas was really persistent in finding out any flaw in every work, but what does the last sentence mean? He didn't allow any perfection in his work? Is it a metaphor? I can't get it well.
ex)...In the late 19th century both armchair theorists and field-workers were studying American Indians and interpreting them in terms of evolutionary schemes. These forces remained opposed to Boas, who advocated scientific skepticism, calling for a return to empirical observation. Boas was aware of this critical character of his method, and described himself as one whose work rests essentially in an unfeeling criticism of his own work and that of others. He was continually self-critical. Flawless perfection must not be sought in Boas.
As he was always criticising himself, then he would find fault with things he had written. It's not a perfect unified work, but something that moves along, gets things wrong, corrects them, changes ideas, etc.