What does the simile I stand in his shadows, silent as a stone mean?

That he does not talk or engage.

Explanation: " Silent as a stone" is referring  to someone who is quiet and doesn't speak there mind.

With regards to human nature versus Modern civilization, Montaigne remains highly skeptical about the capacity of civilization to correct or mitigate the worst aspects of human nature. In accordance to such outlook he considers that only a very healthy and prudent skepticism can allow humans to make the right choices about everything. Because he is skeptical about the actual human capacity to find out what the truth is, let alone understand it, he considers that a measure of emotional detachment is necessary in order to understand reality.

This incapacity of humans to actually find the truth is associated with the philosophical blindness of humans. In this he is verily a man if his time, the Renaissance, during which Greek and Latin Classical Culture was rediscovered in Europe. The Myth of Plato’s Cavern is directly linked to his reasoning on human blindness.

The fact that he opposed European colonization of the America’s is directly related to the opposition between wildness and barbarism: since American natives were considered as “wild” by European colonizers in so much as they lacked the technological advances of the latter and engaged in some barbaric customs such as human sacrifices, the European colonists engaged in barbaric practices that caused great suffering. This is due to the fact that Europeans sought to colonize and civilize these “wild savages” with methods that many times were as barbaric as the practices they were striving to root out.