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Question: When did the classification system start?
Answer: This depends on which classification you have in mind. In science today we have classifications of chemical elements, of stars, and of living things. I assume that your question concerns living things.
As it happens, we can never be certain when something started in history. Some historians could always find an earlier attempt. With living things, the earliest attempt that played a role in further scientific work was that of the Greek philosopher Plato (died in 347 B.C.). He made a classification system of humans, animals and plants. At the lowest level are plants, who have a 'desiring function', then come animals who add to this a 'spirited function', and then people, who add to these a 'rational function'. This scheme is based in the logic of set theory in mathematics that deals with classes and parts of a class known as subclasses. Each subclass adds information to what was in the class. So humans, in the most specific subclass, would have all three functions - desiring, spirited and rational. Humans would be a subclass, or kind of, animals, while animals would be a subclass of plants. Here we need to know that the ancient Greek word for 'plants' was being defined in a much broader sense than we do today - something like 'living things that don't move about.'
Modern biological taxonomy has followed the same logical form, as it was set up by the Swedish botanist, Linnaeus (died 1778), with the categories: Class/Order/Family/ Genus/Species. Here the Class is the basic class and the others are subclasses of it. With increased knowledge of the details of differences between kinds, another basic idea of set theory came into play. As Darwin pointed out the classification system forms a tree. The most general category is the trunk and more specific categories form branches. For example, different related species would form separate branches in a genus. So, in the cat genus we have lions, tigers, cheetahs, bobcats, etc., each on a different branch. Today we consider that there is only one species of modern people - sapiens - assigned to one genus - Homo. We are therefore called Homo sapiens when our genus and species are put together. This type of genus-species way of naming something is used in the biological sciences today. Canis lupis, for example is the genus-species name for the wolf, which tells you it is related to canines or dogs.
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