Reviewer: Many historians claim, in their own treatment of subject matter, to be as little affected as any natural scientist by moral or aesthetic preconceptions.
Conclusion (clearly)
We cannot accept these proclamations of objectivity. rephrase: These historians who are claiming that in their own treatment of subject matter they are unaffected by any moral or aesthetic preconceptions are wrong; their treatment of subject matter is affected by moral / aesthetic preconceptions.
Evidence (for)
It's easy to find instances of false historical explanations embodying the ideological or other prejudices of their authors.
Evaluate
This is a tough argument to understand, but it boils down to this.
This would be like if we argued,
Just because some NBA players cheat on their spouses doesn't mean we should reject the claims of other NBA players who claim that they don't.
Similarly, just because some historians let their biases affect their work doesn't mean we should dismiss the claims of other historians who claim that they don't less bias affect their work.
Goal
Look for an answer saying,