Enumerations count

“There is no definition of what constitutes a planet in our solar system. The only way that I know of now is to enumerate all the planets from Mercury to Pluto”, said Hans Rickman, professor at the department of Physics and Astronomy, Uppsala University, and former secretary-general of the International Astronomical Union (IAU), in 2005. What Hans Rickman had in mind is in terminology science called an “extensional definition”. Extensional definitions are based on the extension of the concept being defined, and Pluto belonged to the extension of the concept “planet” until 2006. When an intensional definition was formulated by the IAU which was based on the intension of “planet”, citing three distinguishing characteristics of the concept. Our perception, Pluto ceased to be considered one of the planets. Our perception of the Universe has changed and with it an extensional definition, “Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto”, had to give way to an intensional definition. That a change in definition changes our perception of the universe is not a new insight, but in this case, it was a change of definition type, from an extensional to an intensional definition. This illustrates the fact that intensional definitions are preferred. Still, extensional definitions are often used alongside intensional definitions. Why, in what situations and what effect does the ordering of their enumerated elements have?