Principle, in chemistry, refers to a historical concept of the constituents of a substance, specifically those that produce a certain quality or effect in the substance, such as a bitter principle, which is any one of the numerous compounds having a bitter taste.
The idea of chemical principles developed out of the classical elements.
Paracelsus identified the tria prima as principles in his approach to medicine.
In his book The Sceptical Chymist of 1661, Robert Boyle criticized the traditional understanding of the composition of materials and initiated the modern understanding of chemical elements.
Georg Ernst Stahl published Philosophical Principles of Universal Chemistry in 1730 as an early effort to distinguish between mixtures and compounds.