A man who does not know what has been thought by those who have gone before him is sure to set an undue value upon his own ideas—ideas which have perhaps been tried and found wanting. As accumulated learning stifles the mental powers, so original thinking has been known to bring about a puffy, unsubstantial mental condition.

Every man has some peculiar train of thought which he falls back upon when he is alone. This, to a great degree, moulds the man.

A man doesn't know what he knows until he knows what he doesn't know.

The will of man has no power whatever over his opinions; he must, and ever did, and ever will, believe what has been, is, or may be impressed on his mind by his predecessors, and the circumstances which surround him.

a man only knows what he's experienced

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.

A man never knows exactly how the child of his brain will strike other people.

A particular train of thought persisted in, be it good or bad, cannot fail to produce its results on the character and circumstances. A man cannot directly choose his circumstances, but he can choose his thoughts, and so indirectly, yet surely, shape his circumstances.

Every man is what he is, because of the dominating thoughts which he permits to occupy his mind.

The lesson of forever and ever is that knowing a man's mind ain't knowing the man.

there are very few who can think, but every man wants to have an opinion; and what remains but to take it ready-made from others, instead of forming opinions for himself?

Man can embody truth but he cannot know it.

A man may not know what will happen as a result of other people’s actions or as the result of unknown causes, but he always knows all possible results of his own actions.

It is certainly true that whatever a man may do or say, the most significant thing about him is what he thinks; and significant also is how he came to think it, why he continued to think it, or, if he did not continue, what the influences were which caused him to change his mind.