Taxonomic categories are useful fictions, not real entities.

Evidence

That it is to the class of fictitious, and not to the class of real entities, that these imaginary, however really useful receptacles, appertain, is, at this time of day, sufficiently clear; but the time has been when they have been mistaken for realities. — Jeremy Bentham, The Panopticon Writings, 531

Categorization

• DEFINE: Classification categories (kingdom, genus, species) are admitted to be useful fictions, once mistaken for realities.
• XREF: A nominalist/constructivist precursor to modern critiques of natural kinds and to nominal-system literature.