"I must maintain a certain pretension, which is far enough from my wish. I must be put on my defence, I must take up the gauntlet continually, or I find I lose ground." by William Hazlitt, from "Essay XXX: On the Disadvantages of Intellectual Superiority" (first published in 1854) Nowadays for "I must be put on my defence," we would say, "I must always go on the defense" or something similar to that. The meaning is "I can't let my guard down" and must always be mentally on high alert. The essay writer was unusually highly educated, more than most of the people around him. He felt he could not relax and enjoy himself in social situations, because people would misunderstand or misinterpret what he said and consequently develop a poor opinion of him. In other words, Hazlitt tried to blend in by covering up his intelligence.

I think you mean "I must BE put on my defence (defense)," in which case the author is probably trying to say that he must be ready at all times, or he must stay alert, or risk a sneak attack of some kind. Perhaps an attack from the intellectually inferior?

Before I jumped to any conclusion, I looked up your quote, and you had written it exactly right. As to what it means, it is poetic, from a long essay, and to me it seems to mean that he must constantly challenge his rival so that his rival will keep the writer from becoming lazy or indifferent.

You have a mistake in your quotation. Maybe there is one word that should not be there. Please check this.