The Sceptical Chymist: or Chymico-Physical Doubts & Paradoxes is the title of a book by Robert Boyle, published in London in 1661. In the form of a dialogue, the Sceptical Chymist presented Boyle's hypothesis that matter consisted of corpuscles and clusters of corpuscles in motion and that every phenomenon was the result of collisions of particles in motion. Boyle also objected to the definitions of elemental bodies propounded by Aristotle and by Paracelsus, instead defining elements as "perfectly unmingled bodies" (see below). For these reasons Robert Boyle has sometimes been called the founder of modern chemistry. 
The first part of the book begins with 5 friends ( Carneades the host and the Skeptic, Philoponus the Chymist, Themistius the Aristotelian, Eleutherius the impartial Judge, and an unnamed narrator) meeting in Carneades's garden and chatting about the constituents of mixed bodies. In part one, Carneades (Boyle) lays out four propositions to the gathering, which sets the foundation for the rest of the book. They are as follows:
Boyle first argued that fire is not a universal and sufficient analyzer of dividing all bodies into their elements, contrary to Jean Beguin and Joseph Duchesne. To prove this he turned for support to Jan Baptist van Helmont whose Alkahest was reputed to be a universal analyzer.
Boyle rejected the Aristotelian theory of the four elements (earth, air, fire, and water) and also the three principles (salt, sulfur, and mercury) proposed by Paracelsus. After discussing the classical elements and chemical principles in the first five parts of the book, in the sixth part Boyle defines chemical element in a manner that approaches more closely to the modern concept:
However, Boyle denied that any known material substances correspond to such "perfectly unmingled bodies." In his view, all known materials were compounds, even such substances as gold, silver, lead, sulfur, and carbon.
According to E. J. Dijksterhuis, "After the appearance of The Sceptical Chymist Aristotle’s doctrine of the four elements as well as Paracelsus’ theory of the three principia gradually passes into disuse." 
Philosopher of science Thomas Kuhn references The Sceptical Chymist in the eleventh chapter of his book The Structure of Scientific Revolutions, while discussing his views on the historiography of science. 
The Sceptical Chymist is referenced in the novel Quicksilver.