While opinion is invisible its effects are anything but. All individual action is driven by opinion. We do nothing unless we are of the opinion that our acting will improve circumstances or prevent them from becoming worse.

And, while we share much in common with our fellows, it is impossible that one person will have identical opinions regarding every topic that another individual has.

The challenge is to make the best use of this reality. We can learn very little from those who agree with us, but a good deal from those intelligent and well informed people who disagree with us. J. S. Mill understood this and used it to eloquently defend free speech in his 'On Liberty', a must read for any free speech proponent.