Quote by John Stuart Mill

No one can be a great thinker who does not recognize that as a thinker it is his first duty to follow his intellect to whatever conclusions it may lead. Truth gains more even by the errors of one who, with due study, and preparation, thinks for himself, than by the true opinions of those who only hold them because they do not suffer themselves to think.

Please suggest source details if you know: book, page, year, online article address (url), correction (different author...), etc.. Thanks.