Answer not. The point of Proverbs 26:3 is that you cannot reason with. fool; Proverbs 26:4 gives the reason. If you answer not according to his folly, he will think he is wise like yourself. If you do answer him according to his folly, he will think you are. fool like he is: i.e. according to Proverbs 26:3 you cannot reason with him. These are finely stated facts, not commands.
folly. Hebrew. 'evil. See note on Proverbs 1:7.