Thoreau writes that people should be "mining themselves for gold." By this he is stating, in another way, the ideal of Socrates, which was to A. put first things first. B. know thyself. C. deny thyself to learn what is valuable. D. live and let live.

The correct answer is B. Know thyself

Socrates believed that people possess all the knowledge of the world, they just can't remember it, so you should use your brain and logic to try to remember this knowledge. This would be knowing thyself and this is what Thoreau also believed.