Benjamin Franklin said this quote

“ [I retained] only the Habit of expressing my self in Terms of modest Diffidence, never using when I advance any thing that may possibly be disputed, the Words 'Certainly, 'undoubtedly', or any others that I give the Air of Positiveness to an Opinion; but rather say 'I conceive', or 'I apprehend a Thing to be so or so', 'It appears to me', or 'I should think it so or so for such & such Reasons', or 'I imagine' it to be so or so, or 'it is so' if I am not mistaken.—This Habit I believe has been of great Advantage to me, when I have had occasion to inculcate my Opinions and persuade Men into Measures that I have been from time to time engag'd in promoting.—And as the chief Ends of Conversation are to inform, or to be informed, to please or to persuade, I wish well meaning sensible Men would not lessen their Power of doing Good by a Positive assuming Manner that seldom fails to disgust, tends to create Opposition, and to defeat every one of those purposes for which Speech was given to us, to wit, giving or receiving Information or Pleasure: For if you would inform, a positive dogmatical Manner in advancing your Sentiments, may provoke Contradiction & prevent a candid Attention. ”