What then is a topic? A topic, in its most generic sense, can be any “thing” whatsoever — a person, an entity, a concept, really anything — regardless of whether it exists or has any other specific characteristics, about which anything whatsoever may be asserted by any means whatsoever. You can't get much more general than that! In fact, this is almost word for word how the topic map standard defines subject, the term used for the real world “thing” that the topic itself stands in for. We might think of a “subject” as corresponding to what Plato called an idea. A topic, on the other hand, is like the shadow that the idea casts on the wall of Plato's cave: It is an object within a topic map that represents a subject. In the words of the standard: “The invisible heart of every topic link is the subject that its author had in mind when it was created. In some sense, a topic reifies a subject...” Strictly speaking, the term “topic” refers to the object or node in the topic map that represents the subject being referred to. However, there is (or should be) a one-to-one relationship between topics and subjects, with every topic representing a single subject and every subject being represented by just one topic. To a certain degree, therefore, the two terms can be used interchangeably.[3]