The man who does not do his own thinking is a slave, and is a traitor to himself and to his fellowmen.

A man without words is a man without thought.

He who is not master of his own thoughts is not accountable for his own deeds.

Men simply don’t think.” And

A man who has the knowledge but lacks the power clearly to express it is no better off than if he never had any ideas at all.

A man is responsible for his ignorance.

He who will not use the thoughts of other men's brains proves that he has no brains of his own.

The most hopelessly stupid man is he who is not aware he is wise.

Do not believe that you alone can be right.
The man who thinks that,
The man who maintains that only he has the power
To reason correctly, the gift to speak, the soul — A man like that, when you know him, turns out empty.

Indeed it may be said with some confidence that the average man never really thinks from end to end of his life. There are moments when his cogitations are relatively more respectable than usual, but even at their climaxes they never reach anything properly describable as the level of serious thought. The mental activity of such people is only a mouthing of clichés. What they mistake for thought is simply a repetition of what they have heard. My guess is that well over eighty per cent. of the human race goes through life without having a single original thought. That is to say, they never think anything that has not been thought before and by thousands.

Intellect is invisible to the man who has none.

A man must be both stupid and uncharitable who believes there is no virtue or truth but on his own side.

No man is happy who does not think himself so.

If a man's at odds to know his own mind it's because he hasn't got aught but his mind to know it with.