A17th-century chemist wrote of the "simple bodies which enter originally into the composition of mixtures and into which these mixtures resolve themselves or may be finally resolved ." what is being discussed?

The correct answer is reactants or reagents of a chemical or physical process.

Explanation:  Chemical reagent is a chemical species used in a chemical reaction. It is usually a chemical substance that is added for the purpose of causing a chemical phenomenon. It is used as a bacteriological dye and as an indicator.