- El Tractatus de amore heroico, (“Treatise on heroic love”), the oldest known work by Arnau, emerged as an epistle that he sent in response to a Sardinian friend. It is the first medical monograph devoted to lovesickness. The author defines “heroic” or passionate love not as an ailment but as an accident, an alteration in the faculty of love caused by a heating of the spirits, which leads the enamoured person to the deception of believing that the beloved is above all others. After explaining the causes of this disorder, he sets forth the signs that allow it to be diagnosed and the way it should be treated before it becomes melancholy and mania, with the consequent risk of death.
- Epistola de reprobatione nigromanticae fictionis (“Epistle on the reproval of the fiction of necromancy”), also known as De improbatione maleficiorum, is an epistle addressed to the bishop of Valencia, most likely Jaspert de Botonac (1276-1288). At its core is a scholastic argument in which Arnau, from the vantage point of natural philosophy, denounces necromancy by denying its fundamental principle: the necromancer’s ability to force a spirit or demon to execute his will in order to ascertain hidden or future events or fulfil his desires. The author seeks to prove that there is no natural or supernatural power within the reach of the necromancers to dominate spirits. In the last part of the epistle, the author takes a medical orientation and reaches the conclusion that those who see necromancy as a rational body of knowledge show that they are mentally ill and suffering from melancholy.
- Tractatus de intentione medicorum (“Treatise on physicians’ intentions”) is apparently the oldest conserved work written in Montpellier. The first part sets forth how philosophical and medical truth can be brought into harmony with each other. The second presents four questions in which Aristotle and Galen seem to disagree: the primacy of the heart over other organs, the neutral state between illness and health and the nature of the soul. The conclusion is that philosophical truth and medical truth do not necessarily have to match, but they are not contradictory given that the physician’s interest to seek the maximum efficacy in his practice of medicine, while the philosopher’s goal is to explore the true nature of whatever they study.
- Tractatus de humido radicali (“Treatise on radical humidity”) was regarded by Arnau himself as a non-medical work. Indeed, even though the topic may have a medical application, the perspective given by the author is more similar to natural philosophy. As he states in the preface, Arnau wrote it with the purpose of clarifying for his colleagues the confusion that he believed some contemporary philosophers were propagating on this concept. The organisation reflects this polemical intention, as the two parts of the treatise show glimpses of its likely origin in two quaestiones disputatae, the name of the scholastic genre which was used in public discussions on given problems within university instruction. The first part defines the concept formulated by Avicenna on radical humidity, an indistinguishable fluid dispersed throughout the entire body grounded upon innate heat and organic life. He also explores its origin: it comes from the sperm that engendered the being but is constantly restored thanks to the process of nourishment. The second part debates whether it is possible to regenerate radical humidity. The conclusion is that radical humidity can be restored up to a certain point, beyond which medicine cannot extend human life, even though it can help it reach its maximum.
- De considerationibus operis medicinae (“On the considerations of the medical practice”), rwritten between 1298 and 1300, begins with a prologue devoted to Gosinus of Cologne and the infant Ferran, son of Jaume II of Mallorca. In it, Arnau clearly sets forth Galen’s doctrine of therapeutic indication and then goes on to discuss practice, using the example of phlebotomy throughout the entire treatise. Counter to mechanical application and based on particular data coming from empirical medicine, the author advises using the rational study of the medical art upheld on universal rules as the basis of evaluating the valid therapy in an individual being, taking all the patient’s circumstances into consideration.
- De dosi tyriacalium medicinarum (“On the dosage of medical theriac”) is a brief treatise in which he once again clashes with Averroes – and with Avicenna as well – when explaining Galen’s claim that the nature of an antidote is halfway between poison and the affected body. It is not until the end of the treatise that he discusses the dosage of theriac, the most highly prized medicine in the Galenist pharmacopeia, made up of a large number of ingredients and used as an antidote and remedy for many poisons. Theriac’s efficacy against poisons is based on its hidden properties, whose strength cannot be determined by experience. Therefore, experience is needed to ascertain the minimum dosage for it to have an effect.
- Speculum medicinae (“Mirror of medicine”, c. 1308) is the work that culminated Arnau’s career as a medical writer. Following the order of the Isagoge by Joannici Ioannitius, a brief introduction traditionally used in medical teaching, it is a synthesis of Galenist theory. Arnau’s purpose in writing it was to offer a systematic compilation of the general principles of the art of medicine to serve as an introduction to the field. In the first few chapters he summarises the basic elements in the human organism and the external factors that affect it. He then surveys the fundamental notions of health and illness and classifies the illnesses. Next he discusses the signs that allow the physician to make a diagnosis. This is followed by an extensive general pharmacology and ends with the organism’s influence on the emotions. Despite its generalist orientation, it reflects the interests that attracted master Arnau throughout his entire career, such as medical instrumentism and the theory of complexions.
- Compilatio de conceptione ("Compilation on the conception"): This is simply a personal schematic synthesis in which he compiles the possible causes of sterility in men and women as a guide for diagnosing it. In the introduction, he briefly explains the structure of this scheme and how it can be used.