The objectivist claim that classical categories exist objectively in the external world is usually taken to be supported by biological evidence. Objectivist philosophers typically point to biological categories like tiger, crow, fish, zebra, etc., which they take as paradigm cases of natural kinds-classical categories that occur in nature and that are defined by essential necessary and sufficient conditions.
- Lakoff (1987), a pag.185

natural kind seems to be part of our folk conception of the world, not part of any scientific conceptual system that there will ultimately be general agreement on. [...[ the concept natural kind plays an absolutely crucial role in objectivist metaphysics.
- Lakoff (1987), a pag.187

Those real discontinuities in nature that are easy for people to perceive-say the differences between elephant and giraffes-correspond to the natural kinds that objectivist cite in justifying their views. The common philosophical examples of natural kinds-tigers, cows, water, gold, etc.-are all basic-level categories in the physical domain.
- Lakoff (1987), a pag.270