The Trialogus is arguably John Wyclif's most influential Latin work, and Stephen Lahey's new translation makes the entire work available for the first time in English. The volume includes a substantial introduction and a translation of On the endowment of the Church, which often accompanied the Trialogus in the manuscript tradition. It will benefit not only those who are interested in Wycliffism but also those with broader interests in the intellectual thought of the fourteenth century. A Latin text intended for lay audiences, the Trialogus spans the normal division of John Wyclif's works into Latin treatises for scholastic audiences and English writings for lay audiences. As such, it is also suitable for teaching fourteenth-century philosophy and theology, subjects where finding texts that are accessible to undergraduates can be difficult. It is easily excerptible, and although the text is not easy, a student who has read Anselm can benefit from seeing how Anselm's arguments continue to be discussed into the fourteenth century.

The conceit of the text is a conversation between Alithia (Truth) and Pseustis (Liar) on the nature of (1) God; (2) the World; (3) Virtue and vice; and (4) the Sacraments and last things. Alithia is a ‘solid philosopher’ who represents a natural theologian, and Pseustis is a friar, who seeks to subvert truth. Their dialogue is mediated by Phronesis (Wisdom), a Christ-figure, who defends truth and pushes the conversation forward towards greater clarity. The organisation of the discourse follows Lombard's Sentences, and the literary form is modelled after a popular Carolingian school text, Theodolus’ Eclogues, with a few distinctive differences. For example, Phronesis plays a greater role in Wyclif's work, and while Pseustis represents pagan mythos in Theodolus, Wyclif's text represents Pseustis as a fourteenth-century friar who uses classical learning and Ockhamist logic to lead the unwitting astray, fostering private religion.

The Trialogus was used by both Wyclif's supporters and opponents as a key to his distinctive doctrines. It grew in popularity in fourteenth-century Oxford and was then read by those who came to be known as the ‘Lollard knights’. The work was cited by name in the ‘Lollard manifesto’ nailed to the door of Westminster Hall and Parliament on 27 January 1395 and in turn responded to by Roger Dymmok op in the Twelve conclusions. It also became the foundation for three other works: the list of errors examined by the Oxford Theology faculty in 1397, the more exact list of errors requested of William Woodford ofm by Arundel, and Thomas Netter's Doctrinale fidei catholicae (1423–30).

Outside of England, the Trialogus was brought to Prague by Bohemian scholars who were interested in refuting the Ockhamism rampant in Charles University, where it became one of the most copied of Wyclif's works and may have been translated into Czech by Jan Hus. Even though early humanists made a printed edition of it in 1525, the work had little influence on the early sixteenth-century reformers, but it experienced a historical revival in the works of Jacques-Bénigne Boussuet (1682), Claude Fleury (1690–1719) and David Hume (1711–76). Gotthard Lechler made the most recent Latin edition of the Trialogus in 1869, well before the Wyclif Society editions, presumably because of its historical importance. An excerpt of the text was translated into English by Robert Vaughn in 1845, but Lahey's is the first full translation of the text.

Lahey's introduction benefits from his earlier biographical work that carefully reconstructs John Wyclif's life and significance. This kind of up-to-date historical research is particularly important in studies of Wyclif, which continue to be pervaded by the on-going repetitions of myths and misinformation. The introduction could have benefited from more editing for flow, but the translation itself is carefully edited and does a good job of making a difficult Latin prose text as readable as possible. Later editions would also benefit from having more substantial footnotes in the translation to help the reader identify the authors with whom Wyclif is engaging, although such a task is not simple given Wyclif's tendency not to name his opponents.

The question of Wyclif's realism and/or Platonism has been of longstanding interest to scholars, especially because it was his rejection of Aristotelian metaphysics that made him sceptical of transubstantiation. One area where Lahey's introduction is somewhat overstated may be his conclusion that John Wyclif was not a Platonist or ‘ultra-realist’ because he did not believe in the existence of universals outside of the mind of God. While I would grant that the label of ‘ultra-realist’ has yet to be defined in a meaningful way among scholars, in light of new developments in the study of medieval Christian Platonism by scholars such as Stephen Gersh, it does not necessarily follow that Wyclif was not a Platonist because there were a variety of Christian appropriations of Platonism, some of which do not hold to ideas existing outside the mind of God. In fact, one of the things that the actual text of the Trialogus highlights well is the changing nature of perceptions of Plato and Aristotle in fourteenth-century scholastic debates over universals, which became increasingly framed by Aristotle's criticisms of Plato in the Metaphysics. Wyclif's Trialogus demonstrates that many philosophers of the time began rejecting Plato's position as prima facie implausible because of its refutation by Aristotle, even though in earlier centuries Plato had been considered the quintessential natural theologian and philosopher whose ideas of God as the Good came the closest to Christian doctrine.