He describes how he cultivated "the habit of expressing myself in terms of modest diffidence; never using, when I advanced any thing that may possibly be disputed, the words certainly, undoubtedly, or any others that give the air of positiveness to an opinion; but rather say, I conceive or apprehend a thing to be so and so; it appears to me, or I should think it so or so, for such and such reasons; or I imagine it to be so; or it is so, if I am not mistaken."

This sounds eminently reasonable but objectively it doesn't seem to be a very effective approach. If you look at people whose success depends on persuading others they rarely take this approach. Politicians, advertisers, media 'experts', preachers and those peddling all manner of quack cures and schemes rarely display uncertainty or lack of conviction in their own opinions. It seems to me that if such an approach were actually the best way to convince the majority of people then we would see a world where political speeches, adverts, religious sermons and the promotion of alternative medicine relied on 'expressing oneself in terms of modest diffidence'. I don't see such a world.