Two weeks ago I wrote about the problem with definitions. At scienceblog.com, this post got over 11,000 hits and 41 comments, most of which had to do with answering the age-old challenge of defining the word “chair.” There were some very good attempts, none of which ultimately work, which isn’t surprising since may of the greatest minds in the 20th century have tried and failed to solve this problem.
We do not wish to have a concept for every single object–such concepts would be of little use and would require enormous memory space. Instead, we want to have a fairly small number of concepts that are still informative enough to be useful (Rosch 1978). The ideal situation would probably be one in which these concepts did pick out objects… Unfortunately, the world is not arranged so as to conform to our needs.
For example, it may be useful to distinguish chairs from stools, due to their differences in size and comfort… However, there is nothing to stop manufacturers from making things that are very large, comfortable stools; things that are just like chairs, only with three legs; or stools with a back. These intermediate items are the things that cause trouble for us, because they partake of the properties of both…The gradation of properties in the world means that our smallish number of categories will never map perfectly onto all objects: The distinction between members and nonmembers will always be difficult to draw or will even be arbitrary in some cases.