 The Millian view applied to simple natural kind terms faces various problems. One of these problems concerns empty terms. ‘Phlogiston’, ‘caloric’, ‘ether’, ‘humour’, are all natural kind terms that fail to refer to anything. Millianism implies that there is no semantic difference between these terms at all. Surely ‘phlogiston does not exist’ and ‘caloric does not exist’ mean different things; Lavoisier asserted the first and denied the second with apparent consistency. The most common response to this problem is a retreat from pure Millianism for such terms. Some descriptive element is included in their semantics.