Excerpt from two entries of the dictionary.
"It is with astonishment, no doubt, that one will see that we now accept as the principles of all compounds the four elements, fire, air, water and earth, indicated as such by Aristotle very long before one had the chemical knowledge needed to discover such a truth. In fact, however one decomposes substances, one can only ever obtain these substances: they are the final terms of chemical analysis."
"It is very possible that these substances, although thought to be simple, may not be so, that they may even be highly compound, resulting from the union of several principles, other simple substances, or that they are transmutable one into another, as is thought by M. le compte de Buffon. But as experiment teaches us absolutely nothing about this, one may without any inconvenience regard, and in chemistry even must regard, fire, air, water and earth as simple substances; because in fact they act as such in chemical operations."
Bensaude-Vincent, B. (1995) Lavoisier: A Scientific Revolution. In: M. Serres (editor): A History of Scientific Thought, Elements of a History of Science. Blackwell, Oxford, 455 - 482. (Translation of Éléments d'Histoire des Sciences, Bordas, Paris, 1989)