Answer not. The point of Pro_26:3 is that you cannot reason with a fool; Pro_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 a fool like he is: i.e. according to Pro_26:3 you cannot reason with him. These are finely stated facts, not commands.