more from Alexander Bird

Single Idea 6767

[catalogued under 26. Natural Theory / B. Natural Kinds / 1. Natural Kinds]

Full Idea

Both rubies (valuable) and sapphires (less valuable) are corundum (Al2O3), differing only in their colours, for which traces of iron, titanium and chomium are responsible.

Gist of Idea

Rubies and sapphires are both corundum, with traces of metals varying their colours


Alexander Bird (Philosophy of Science [1998], Ch.3)

Book Reference

Bird,Alexander: 'Philosophy of Science' [UCL Press 2000], p.106

A Reaction

A nice example which illustrates how natural kinds determined by nominal essence could be drastically different from those suggested by real essence. It certainly suggests that corundum might be a natural kind, but ruby isn't.