自相矛盾 (Zìxiāngmáodùn), or roughly, “the spear against the shield”, is an idiom that is used to describe things that are self-contradictory.
Once upon a time, a cocky weapons salesman stood in the marketplace and advertised his goods: “My spear is the sharpest in the world”, he shouted proudly, loudly and salesmanlike, “and it can go through anything!”
People stopped to listen as he continued: “And my shield is the strongest in the world – nothing can break it!”
All this obviously sounded quite good, and people were excited about these treasures, until one little boy remarked: “So – what if you use your spear against your shield?”
So whenever people claim two things that can’t be true at the same time, or if they act paradoxically, you can gently inform them that you're in a spear-and-shield situation.
Now here’s a spear-and-shield situation… Flickr