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Binomial nomenclature. To indicate whether they think a given type of organism is a "species," biologists use binomial nomenclature, which Webster's Third International Dictionary defines as
Unfortunately for modern scientists, these names are usually in Latin or Greek. But this system was devised during the first half of the eighteenth century, when the educated were still masters of those languages and Aristotelian logic yet held sway. Although it seems unlikely that anyone would come up with such a system today, it must have seemed quite natural and tidy to most naturalists in the days when it was invented.
Because all existing classifications of living organisms were cast in terms of binomial nomenclature when biology first began, scientists kept using the same system, even after the days of the Scholastics had been left far behind and the task of biological classification had become a secular activity. Moreover, the word species remained as prominent in scientific debate as it had been in the religious and philosophical discussions of old. Naturalists had always used the term and they continued to do so. In constructing their systems of classification, the schoolmen had wanted to be sure that they were correct when they called a particular type of plant or animal a "species." Modern scientists who construct such systems (taxonomists) want to be sure of the same thing.
Debate has been endless over the question of what exact criteria are appropriate for making such determinations. It sometimes seems as if nothing has changed in the several centuries since John Jewel remarked on his youth among the Oxford logicians: "What adoo was made in daily disputations for exercise of young wittes, aboute Genus and Species, and the reste of the Universals" (A Defence of the Apologie of the Churche of Englande, 1567).
For the outsider, the terminology and reasoning of a modern taxonomist can be every bit as arcane as that of the Scholastics. But we'll only need to cover two points for the purposes of subsequent discussion:
A biologist says any population assigned a trinomial is "treated as a subspecies." Webster's defines a trinomial as
When two populations or individuals are assigned the same binomial they are "treated as conspecific."
Thus, if biologists are to use binomial nomenclature, and if they are to carry out the process of naming an organism in an objective manner, they need a clear definition of species. But attempts to say which populations should be treated as species have always been, and continue to be, problematic. In the long quest to establish valid classifications, a multiplicity of definitions for species have been proposed. This hunt for definitions is of such long standing, that it is known in biological circles as "the species problem."