Chalmers believed that Paine's arguments are pure fantasy that would be harmful to the colonies, as to him independence is a ridiculous idea. It would open them up to the French and the Spaniards and lose the them the most valuable trading partner which was at the time England.
I believe the correct answer is naive. The author is trying to say that he was ignorant, and that he couldn't understand properly these new revelations. He was either too young or too stupid to understand such a thing, but he believes he will as he grows older.