TheOpus Majus (Latin for "Greater Work") is the most important work ofRoger Bacon. It was written inMedieval Latin, at the request ofPope Clement IV, to explain the work that Bacon had undertaken. The 878-page treatise ranges over all aspects of natural science, from grammar and logic to mathematics, physics, and philosophy. Bacon sent his work to the Pope in 1267.^[1] It was followed later the same year by a smaller second work, hisOpus Minus, which was intended as an abstract or summary of the longer work, followed shortly by a third work,Opus Tertium, as a preliminary introduction to the other two.

TheOpus Majus is divided into seven parts:

1. Part one considers the obstacles to real wisdom and truth, classifying the causes of error (offendicula) into four categories: following a weak or unreliable authority, custom, the ignorance of others, and concealing one's own ignorance by pretended knowledge.
2. Part two considers the relationship betweenphilosophy andtheology, concluding that theology (and particularlyHoly Scripture) is the foundation of all sciences.
3. Part three contains a study of Biblicallanguages:Latin,Greek,Hebrew, andArabic, as a knowledge of language and grammar is necessary to understand revealed wisdom.
4. Part four contains a study ofMathematics: As part of the study, he vividly drew out the flaws in theJulian Calendar, proposing to drop a day every 125 years from 325 CE (Council of Nicaea). He also noted the shifting of theEquinoxes to theSolstices.^[2]
5. Part five contains a study ofOptics: The study of optics in part five seems to draw on the works of the Arab writersKindi andAlhazen, including a discussion of thephysiology ofeyesight, theanatomy of theeye and thebrain, and considerslight, distance, position, and size, direct vision,reflected vision, andrefraction,mirrors andlenses.
6. Part six,De scientia experimentalis, a study ofExperimental Science: It includes a review ofalchemy, the creation of gunpowder and of the positions and sizes of thecelestial bodies, and anticipates laterinventions, such asmicroscopes,telescopes,spectacles,flying machines,hydraulics andsteam ships. Theoccult overtones of this section reflect Bacon's interest inmagic, which he also wrote about inDe secretis operibus artis et naturae, et de nullitate magiae. It was a major influence onJohn Dee's theory ofArchemastrie.^[3]
7. Part seven considers moral philosophy andethics.

An incomplete version of Bacon'sOpus Majus was published byWilliam Bowyer inLondon in 1733. It was edited bySamuel Jebb from a manuscript atTrinity College, Cambridge which omitted the seventh part.

As a recent paper emphasizes, this major work cannot be usefully read exclusively in the context of the history of science and philosophy while forgetting to consider Bacon's religious commitment to theFranciscan Order. "His Opus maius was a plea for reform addressed to the supremespiritual head of the Christian faith, written against a background ofapocalyptic expectation and informed by the driving concerns of the friars. It was designed to improve training formissionaries and to provide new skills to be employed in the defence of the Christian world against the enmity of non-Christians and of theAntichrist".^[4]

• A History of Western Philosophy Vol. II by Ralph McInerny from the Jacques Maritain Center,University of Notre Dame
• Roger Bacon from theCatholic Encyclopedia
• Fr. Rogeri Bacon Opera quaedam hactenus Inedita, Vol. 1 at Google Books. Contains theOpus Tertium,Opus Minus, andCompendium Philosophiae.

• Opus Majus, Volume I in the Internet Archive – original text in Latin (omitting Part IV), ed. by John Henry Bridges, 1897.
• Opus Majus, Volume I in the Internet Archive – original text in Latin (includingPart IV), ed. by John Henry Bridges, 1900.
• Opus Majus, Volume II in the Internet Archive – original text in Latin, ed. by John Henry Bridges, 1897.
• Opus Majus, Volume I at Google Books – English trans. by Robert Belle Burke, 1928.
• Opus Majus, Volume II in the Internet Archive – English trans. by Robert Belle Burke, 1962.

• Works by Roger Bacon
• 1267 books
• British literature
• 13th-century books in Latin
• Medieval philosophical literature

• Articles containing Latin-language text
• Articles with short description
• Short description is different from Wikidata