Concrete:
* When referring to physical objects: "The sights of the city were amazing, especially the Eiffel Tower." Here, "sights" refers to specific landmarks and structures, which are physical objects.
* When referring to the sense of sight: "The old man's sights were failing." Here, "sights" refers to the ability to see, which is a physical capacity.
Abstract:
* When referring to a general concept of things seen: "The tour guide showed us the sights of the ancient city." Here, "sights" refers to the general idea of interesting things to see, not specific objects.
* When referring to a visual experience: "The sights of the Grand Canyon were breathtaking." Here, "sights" refers to the experience of seeing the Grand Canyon, which is an abstract idea.
Therefore, the context determines whether "sights" is abstract or concrete.