Alexander of Hales (also Halensis, Alensis, Halesius, Alesius /əˈliːsiəs/; c. 1185 – 21 August 1245)—known also as Doctor Irrefragibilis, or "Irrefutable Teacher" (so-called by Pope Alexander IV in the bull De Fontibus Paradisi), and as Theologorum Monarcha (or "King of Theologians")—was a Franciscan friar, theologian, and philosopher instrumental to the development of scholasticism.

During the University strike of 1229, Alexander participated in an embassy to Rome to discuss the place of Aristotle in the curriculum.

[4] The latter title is especially suggestive of his role in educating several Franciscans who later became influential thinkers in the faculty, among them Bonaventure, John of La Rochelle, Odo Rigaldus, William of Middleton, and Richard Rufus of Cornwall.

When using the works of his authorities, Alexander does not only review their reasoning but also gives conclusions, expands on them, and offers his agreements and disagreement with them.

He also differed in that he appealed to pre-Lombardian figures, and by referencing Anselm of Canterbury and Bernard of Clairvaux, whose works were not cited as frequently by other 12th-century scholastics.

While it is common for scholars to state that Alexander was the first to write a commentary on the Sentences of Peter Lombard, it is not quite accurate.

[8][incomprehensible] Of Alexander's Summa, which was on one occasion proclaimed by an assembly of seventy doctors to be infallible, Roger Bacon declared that, though it was as heavy as the weight of a horse, it was full of errors and displayed ignorance of physics, of metaphysics, and even of logic.

[4] This steered the development of scholasticism in a more systematic direction,[4] inaugurating an important tradition of writing commentaries on the Sentences as a fundamental step in the training of master theologians.

In doing so, he elevated Lombard's work from a mere theological resource to the basic framework of questions and problems from which masters could teach.

The commentary (or more correctly titled a Gloss) survived in student reports from Alexander's teaching in the classroom and so it provides a major insight into the way theologians taught their discipline in the 1220s.

As is the case with Glossa and Quaestiones Disputatae, much of his work is probably written in the form of notes on his oral teachings by students, though the content is definitely his.

It treats in its first part the doctrines of God and his attributes; in its second, those of creation and sin; in its third, those of redemption and atonement; and, in its fourth and last, those of the sacraments.

This massive text, which Roger Bacon would later sarcastically describe as weighing as much as a horse, was unfinished at his death; his students, William of Middleton and John of Rupella, were charged with its completion.

While there was a ban on using Aristotle's works as teaching texts, theologians like Alexander continued to exploit his ideas in their theology.

[19] Specifically, in one of his last works, De scientia divina, he concludes that the idea of plurality itself is strictly temporal, a human notion.

[21] Just cause becomes the overarching moral principle for declaring war in three ways: the relief of good people, coercion of the wicked, and peace for all.