A treatise is a formal, lengthy, systematic discourse on some subject.

Noteworthy treatises

• Treatises have been written by various philosophers:
• Aristotle—various treatises
• Xenophon—Oeconomicus
• Chāṇakya—Arthashastra
• Claudius Ptolemaeus—Almagest
• John Locke—Two Treatises of Government
• David Hume—A Treatise of Human Nature
• René Descartes—Treatise on the World and Compendium Musicae
• William Godwin—one of the first anarchist treatises
• Karl Marx—Das Kapital
• Niccolò Machiavelli—The Prince, and Discourses on Livy
• Hector Berlioz—Treatise on Instrumentation (sometimes, Treatise on Orchestration)
• Charles Darwin—The Origin of Species
• Nizam al-Mulk—Siyasatnama (The Book of Government)

• Other well-known treatises include:
• Sun Tzu—The Art of War
• Thomas Paine—Rights of Man, Common Sense, and The Age of Reason
• Geoffery Chaucer—The Parson's Tale
• Euclid—Elements

• Recent treatises in the natural sciences include:
• Treatise on Invertebrate Paleontology, Volumes A through W, edited by Raymond C. Moore, Roger L. Kaesler, and others (1953 to 2006, and continuing). Lawrence, Kansas: University of KansasPress; and Boulder, Colorado: Geological Society of America. -- Two recent volumes are, for example, ISBN 0813731356 (Brachiopoda, Revised, Volume 5, published 2006) and ISBN 0813731313 (Porifera, Revised, Volume 3, published 2004).
• Treatise on Marine Ecology and Paleoecology, Volume 1: Ecology and Volume 2: Paleoecology edited respectively by Joel W. Hedgpeth and Harry S. Ladd (1957/1971). Washington, D.C. : WaverlyPress; and Boulder, Colorado: Geological Society of America.

• Recent economics treatises have also been written:
• Paul Samuelson—Foundations of Economic Analysis
• Milton Friedman—Capitalism and Freedom

• Treatises on architecture:
• De architectura, Vitruvius (ca. 40 CE)
• De re aedificatoria, Leon Battista Alberti (1450)
• I Quattro Libri dell'Architettura, Andrea Palladio (1570)

• EU Energy and transportation:
• Treatise project provides free training in environmental transport for energy, environmental and fleet professionals 1.