Apology (Apologia Socratis), Plato

… if you kill me you will not easily find a successor to me, who, if I may use such a ludicrous figure of speech, am a sort of gadfly, given to the state by God; and the state is a great and noble steed who is tardy in his motions owing to his very size, and requires to be stirred into life. I am that gadfly which God has attached to the state, and all day long and in all places am always fastening upon you, arousing and persuading and reproaching you. You will not easily find another like me, and therefore I would advise you to spare me. I dare say that you may feel out of temper (like a person who is suddenly awakened from sleep), and you think that you might easily strike me dead as Anytus advises, and then you would sleep on for the remainder of your lives, unless God in his care of you sent you another gadfly. When I say that I am given to you by God, the proof of my mission is this:— if I had been like other men, I should not have neglected all my own concerns or patiently seen the neglect of them during all these years, and have been doing yours, coming to you individually like a father or elder brother, exhorting you to regard virtue; such conduct, I say, would be unlike human nature. If I had gained anything, or if my exhortations had been paid, there would have been some sense in doing so; but now, as you will perceive, not even the impudence of my accusers dares to say that I have ever exacted or sought pay of any one; of that they have no witness. And I have a sufficient witness to the truth of what I say – my poverty.

Plato. Apology, Part I

Socratic Wisdom

‘I am wiser than this man; it is likely that neither of us knows anything worthwhile, but he thinks he knows something when he does not, whereas when I do not know, neither do I think I know; so I am likely to be wiser than he to this small extent, that I do not think I know what I do not know.’

Socrates, Apology, Part I

… free and daring speculation …

A state of things in which a large portion of the most active and inquiring intellects find it advisable to keep the genuine principles and grounds of their convictions within their own breasts, and attempt, in what they address to the public, to fit as much as they can of their own conclusions to premises which they have internally renounced, cannot send forth the open, fearless characters, and logical, consistent intellects who once adorned the thinking world. The sort of men who can be looked for under it, are either mere conformers to commonplace, or time-servers for truth, whose arguments on all great subjects are meant for their hearers, and are not those which have convinced themselves. Those who avoid this alternative, do so by narrowing their thoughts and interest to things which can be spoken of without venturing within the region of principles, that is, to small practical matters, which would come right of themselves, if but the minds of mankind were strengthened and enlarged, and which will never be made effectually right until then: while that which would strengthen and enlarge men’s minds, free and daring speculation on the highest subjects, is abandoned.

John Stuart Mill, Philosopher, 1806-1873
Of the Liberty of Thought and Discussion
On Liberty, Chapter II