The word <em>vigorous</em> <em>affects the meaning of the sentence by making it clear that the one that says I know, instead of I think is considered as a person who has a strong will and personality.</em>
Explanation:
<u>The word </u><u><em>vigorous</em></u><u> describes something strong, something energetic and forceful</u>; that is why <u>it affects the sentence by expressing how determined is a pilot who uses the phrase </u><em><u>I know</u></em><u>.</u> How decided he is and how strong will he has, how vigorous he is. On the other hand, these lines also show with what scorn a pilot was looked upon if he ever ventured to say <em>I think.</em> This phrase <em>(I think)</em>, in contrast with <em>I know</em>, is a weak phrase which is supposed to transmit insecurities and weak will.