The opposite or antonym for "art of opinion" could be "science of fact." While the former is about expressing subjective thoughts and beliefs, the latter is centered on objective truths and empirical evidence. The art of opinion involves creativity, interpretation, and personal views, while the science of fact emphasizes research, logic, and proof. The two concepts are distinct, but they can complement each other in many ways. A good argument combines the art of opinion with the science of fact to make a persuasive case. By understanding the nuances and differences between these opposites, we can better appreciate the different ways of thinking and communicating.