Answer:
An argument may be either speech or writing that consists of 3 parts. They are Challenge, Defend and Qualify.
Challenge: It is to disagree with the given opinion against any issues, sayings, and thesis of an author.
Defend: It is the state when the author's opinion is correct in an argument and you're agreeing with him.
Qualify: In this state, you can agree only a few parts of the author's opinion and disagree with some of the other parts in the argument. Qualify can be the combination of Challenge and Defend. In the Essay, the author is defending, since Stevenson agreed that success cannot be achieved by staying quiet. By the phrase in the second line saying he has no doubt in it.