Stubborn and ardent clinging to one's opinion is the best proof of stupidity. Michel de Montaigne
The difficult part in an argument is not to defend one's opinion but rather to know it. André Maurois
It often takes more courage to change one's opinion than to keep it. Willy Brandt
The duty and the right to express one's opinion freely may be meaningless-and on occasion even injurious to the best interests of the society-if those opinions are not based on sound thought, which, in its turn, presupposes the possession of sound knowledge. Muhammad Asad
Argument is generally waste of time and trouble. It is better to present one's opinion and leave it to stick or no as it may happen. If sound, it will probably in the end stick, and the sticking is the main thing. Samuel Butler (novelist)
To give a person one's opinion and correct his faults is an important thing. It is compassionate and comes first in matters of service. But the way of doing this is extremely difficult. Yamamoto Tsunetomo