A definition should not use negative terms unnecessarily.
Negative definitions should be avoided because knowing what a thing is doesn't tell us much about what it is.
At the turn of the century, the automobile was described as a "horseless carriage." The differentia "horseless" tells us about one source of power that automobiles do not use. However, there are many sources of power automobiles do not use; what we want to know is the source they do use.
Some concepts, however, are inherently negative, and thus require negative terms in their definitions.
A "bachelor" is a man who is not married.
This has to be defined negatively, since it is an inherently negative concept.
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