Since direct perception depends on effects in a medium, the same effects can be sent from a different source artificially, that is, from pictures, models and other recordings or copies, producing indirect perception. The elements are arranged in a pattern copying the form of the object to some degree. Some objects indicate other objects, by convention or other means, without giving impressions of the other object. Signs can indicate other objects in this fashion. Many signs in nature include spoor, tracks, bedding or nests, kill and other objects caused by an object.
Some signs are entirely conventional e.g. RR meaning railroad.
Linguistic signs deserve a special mention. They include many kinds of referents besides common objects. Notably they include instructions or indications about operations or relations. We can examine the referents of words standing for common objects, and learn a great deal about cows, Spam, and the non-existence of Ugandan mountain whales, or Robertidas that interrupt the solitary peering of astronomers by chewing carrots at the big end of telescopes. Rather, these logical terms indicate how to treat other terms.