AILLY Pierre d' (Pierre Marguerite, Petrus de Alliaco) — philosopher, theologian, b. 1351 in Compiègne, d. 1420 in Avignon.

Pierre d'Ailly studied grammar, philosophy and theology at the College of Navarre in Paris from 1364-1381. He became master of the college in 1384 and held this post until 1389. He lectured up to 1395, then resigned from lecturing in favor of his disciple, John Gerson. He was involved in university disputes and political battles. In 1381 he fought with Hugo Aubriot, who was responsible for order and the judiciary in Paris. Aubriot was ultimately condemned for life. From 1384 to 1386 he fought against the chancellor of the university, John Blanchard who was accused of accepting bribes and had to leave his position. In 1387–1389 he engaged in polemics with John de Monzon and consequently with the Dominicans in general concerning the dogma of the Immaculate Conception. As a result of this dispute, Pierre d'Ailly was made Chancellor of the University and Almoner of the King. The Dominicans thereby lost this office and were expelled from the university for a quarter century.

From 1381 onward, Pierre d'Ailly propagated the idea of assembling a general council to end the so-called Western Schism. At that time he wrote a diatribe, Epistola diaboli Leviathan, in which he attacked those who opposed the idea of a council. D'Ailly's career in the Church was an extension of his university career. He was in turn bishop of Le Puy (1394), Noyon (1396) and Cambrai (1397). As Bishop of Cambrai he was also the Count of Cambrésis and a prince of the Empire. His appointment was against the wishes of the Prince of Burgundy, and this eventually caused the Prince to align himself with the Armagnacs. In 1400, Pierre d'Ailly went to the Council of Pisa which elected a new Pope. This was at the height of the period of the Western Schism in the Church, and at that time there were three popes: a Roman Pope, an Avignon Pope and a Pope in Pisa. In 1411, the Pope, John XXIII, named him a cardinal, and two years later made him Legate in the Empire. D'Ailly took part in the Council of Constance (1414–1418) where he took part in the works of many commissions and contributed to the death of John Hus and Jerome of Prague. He went from Constance to Avignon where he died.

Pierre d'Ailly was one of the most famous figures of his time. His ambitions were matched by his talents, and his talents had been recognized by his two protectors, Cardinal John de la Grange, who was an influential advisor to Carol V and an eminent statesman and the real organizer in the election of an anti-pope in 1378, and Philip de Mézières, who was a poet, diplomat and friend of Nicholas Oresme. In philosophy and theology, Pierre d'Ailly is loosely associated with nominalism, but he remained open as well to Platonism and to the influences of French and Italian humanism.

About two hundred works of different rank in Latin and French are attributed to Pierre d'Ailly. His works concern a wide range of philosophical, scientific, theological and religious problems and belong to various literary genres. L. Salembier in the Dictionnaire de théologie catholique (P 1903, I 642-654) presents the most complete list of d'Ailly's works. P. Glorieux in the Oeuvre littéraire de Pierre d'Ailly (Mélanges des Sciences religieuses 22 (1965), 61-78), made some important additions. The most important manuscripts are preserved in Paris in the Bibliotheque de l'Arsenal (520) in the Bibliotheque Nationale de France (lat. 3122): these manuscripts were written at the behest of d'Ailly and bear his corrections.

There is no edition of the collected writings of d'Ailly. Many are only accessible in manuscripts or in incunabala. Theological questions, some opuscula against astrology and some polemical treatises are listed by L.E. du Pin in Joannis Gersoni Opera omnia (Anvers 1805, I 489-693, 697-804, and ecclesiastical treatises in II 24-32, 74-75, 105-113, 867-960). In Tractatus et sermones (Str 1490, repr. F 1971) a few biblical commentaries, some spiritual writings and many discourses have been published. A portion of his minor writings, often with abbreviations, has been published by P. Tschackert under the title Peter von Ailly (Gotha 1877, repr. A 1968).

In addition to the above-mentioned sources, omitting his French texts, letters, poems, sermons and discourses, we may mention the following. A small collection of his philosophical writings is found in the Tractatus de anima, which was published before 1381 (Str 1490, A 198710). His Tractatus super de consolatione philosophiae Boethii is made up of two questions and was written in the years 1377–1381 (the first question was published in A-Ph 1993). There are three writings on semantics: Conceptus (the first edition was printed as incunabula without place or date, Mr 198011). His works Insolubilia and Tractatus exponibilium (P 1494, 1496-14973), and Super Libros meteororum (Str 1504, Kr 15154) have not appeared in a modern edition. The treatise, Destructiones modorum significandi has been ascribed to d'Ailly but is inauthentic.

His scientific writings were written mainly in the years 1410–1414; they include geographical and cosmological compilations: Imago mundi, Epilogus Mappaemundi and two condensed versions of Ptolemy's Cosmography (published together, I-III p 1930), a few short works on the reform of the calendar, e.g., Exhortatio super Kalendarii correctione (Ve 1875), and a series of works concerning astrology, astronomy and their connections with theology. Of these latter works, the work Secunda apologetica defensio astronomiae veritatis (Au 1490, Ve 1494) was written against Nicholas Oresme's Tractatus contra astrologos. Peter d'Ailly knew and criticized other works by the same scholar, but is not always of the same opinion. In some such works (De falsis prophetis, De legibus et sectis) he minimizes the influence of the stars on the course of events, while in other works he considers this influence to be great, even upon religions. The latter works include De concordia astronomicae veritatis cum theologia and Elucidarium astronomicae concordiae cum theologica et historica veritate (Au 1490, Ve 1494).

A great number of his theological writings are scholastic scriptural commentaries (Expositio super Cantica canticorum with a prefatory lecture Descriptio imaginariae visionis de horto s. Scripturae in 1374, a recommendation of Sacred Scripture and a prefatory lecture to the Gospel of St. Mark in 1375), and commentaries written for use outside of the schools (Verbum abbreviatum super Psalterio, Tractatus super septem psalmos poenitentiales published in Tractatus et sermones (Str 1490), a commentary on books I, III, and IV of Peter Lombard's Sentences along with principles, questions discussed before his doctorate, and two questions from his doctoral examination (both published Str 1490, repr. F 1968). These works also include two speeches against the Chancellor John Blanchard (publish Lei 1987), and against John de Monzon (An 1706, P 1936) — the first speeches concern not only the teaching of theology and the way degrees are recognized in the department, but also the conception of the theologian in the Church. The second speeches are important because of their wealth of topics: the hierarchy of theological authorities (loci theologici), the linguistic rules that hold in theology, the relation of theology to philosophy, and the authority of Thomas Aquinas. An abbreviated version of Ockham's Dialogue comes from the early period of d'Ailly's works, and it is the most important souce for Ockham's ecclesiology. The Epistola diaboli Leviathan (Gotha 1877) is the first public reaction to the double conclaves of 1378. In this diatribe, d'Ailly demands that a council be assembled to end the Western Schism. He supports this point of view in Propositiones utiles ad exterminationem praesentis schismatis per viam concilii generalis (P 1733), which was sent to the cardinals gathered in Pisa, and then in Apologia concilii Pisani in 1412. Other treatises and minor works on this topic were written (partly under Gerson's influence) after the Council of Pisa and at the Council of Constance. The most important of these works is De materia concilii generalis (NH-Lo 1964), De persecutionibus ecclesiae (pub. N. Valois), Bibliothèque de l'École des Chartres 55 (1904), 557-574).

Peter d'Ailly's philosophical thought focuses mainly on semiotics. In his time, semiotics was composed of logic in the broad sense, but he was not especially interested in logic or linguistics, but in epistemology, which he treated in a semantic manner. He regarded meaning and knowledge as closely related concepts. Meanings are categorematic and syncategorematic words (terms), which are thought, pronounced and written according to the triple division of Boethius. Since spoken and written words are synonymous, the fundamental difference concerns the first two members: the thought word is the concept or the mental act of knowledge, and the spoken word is a sign of the conventional meaning. Thereby the term is a sign "which [in reference to the spoken word] by actual convention or [in reference to the concept] by its nature is capable of signifying — for the cognitive power, which it changes vitally — some object (aliquid), some objects (aliquae), or in some way (aliqualiter)", where to signify is to "represent some object or some objects or in some way before the cognitive power, vitally changing it," (Conceptus, 81).

Leaving to the side the traditional distinction between natural and conventional signs, the above definition provides two fundamental and original resolutions to semantic and epistemological problems: the distinction between the signification of categorematic and syncategorematic names, and the assimilation of signification and meaning in the concept of vital modification (immutatio vitalis). In the first matter, the often used expression "aliquid vel aliqua vel aliqualiter" signals that "aliquid" expresses what a categorematic name signifies when it is used in the singular, while "aliqua" expresses what a categorematic names signifies when it is used in the plural, or what a collective name signifies, and "aliqualiter'" expresses what a syncategorematic expression signifies. In reality, d'Ailly makes a distinction between purely categorematic names, purely syncategorematic expressions, and those which are partially categorematic and syncategorematic, such as quantifiers used in the neuter. These mixed expressions may be treated in an analysis as either categorematic or syncategorematic. In thought, their counterparts are composite expressions, e.g., to the expression "aliquod ens" corresponds to the expression "aliquod", i.e., "aliquod ens" is a concept composed of a syncategorematic act and a mental categorematic name. D'Ailly broke with tradition and held that syncategorematic expressions may signify something: they do not, as others supposed, merely modify the signification of judgments.

A mental name or concept is at the same time an act of knowledge and a sign that represents a known object. In essence, "concept", "act of intellectual knowledge", "intellectual apprehension of a thing", "to signify" and "to represent" are interchangeable terms. This assimilation of the two areas is possible because of the conception of vital modification, which is most important in definition and which d'Ailly borrowed from John de Ripa whom d'Ailly severely criticized in Paris, and from Gerson. The meaning that d'Ailly gives to this concept is not exactly the same as what John de Ripa regarded as vital modification. By vital modification, d'Ailly understood an actual act of knowledge present in the cognitive faculty and partially caused by the same faculty. In other words, in order for the actual defined act of knowledge to change the faculty in a vital way, it must meet two conditions: 1) the act of knowledge must be, albeit partially, caused by the cognitive faculty, which implies that the cognitive faculty has an active role in the act of knowledge, and 2) the actual act of knowledge must belong to the faculty in the same way that an accident belongs to a substance. This approach to the concept is important for the typology of signs, since the first division of signs divides signs into those of language (conventional signs) and natural signs (concepts). The former enable us to the objects of which they are signs, and the latter are the very knowledge of these objects and their natural similarity (similitudo).

D'Ailly's semantic and epistemological interests influenced the shape of his theology in two ways. First, they made his theology sensitive to the formulation and resolution of various problems: the norms of theological language established by the Fathers of the Church (e.g., Augustine, Hillary, Pseudo-Dionysius); the basic difference between language and argumentation in theology and language and argumentation in other disciplines such as philosophy (d'Ailly was a vigorous supporter of the separation of disciplines); the rules proper to theology and its own "logic", which depends upon the material being studied. Because of discipline or object studied (materia subiecta), a judgment regarded as true in other sciences may be regarded as false in theology, and vice versa. John Gerson accepted and strongly emphasized this complex of principles in his theory. The second way d'Ailly's semantic and epistemological interested affected his theology was that they made it sensitive to logical instruments (e.g., the distinction between sensus divisus and sensus compositus), and to new conceptual languages (e.g., to an analysis secundum imaginationem that frequently resorts to a mode of reasoning de potentia Dei absoluta) which allowed him to formulate theses or construct logically possible models. Philosophical experience, which consists in the exploration of what is logically possible, expanded the heuristic possibilities of the physical world.

In astronomy and astrology, Peter d'Ailly was less original. He often repeated what his predecessors had said. His efforts were directed to proving on the basis of established truths that events in the sublunary world depend upon the influence of the stars, and that this dependence allows us to foresee future events in some cases. In this context, his interest in the fact, nature and kinds of prophecy grew. Old historians credit d'Ailly with predicting the French Revolution, even the year of its occurrence. They also claim that Christopher Columbus studied d'Ailly's Imago mundi prior to his discovery of America. He predicted that the Antichrist would come and that the end of the world would begin in the year 1789, based on faulty calculations of the cycles of Saturn that began at an arbitrary point. As for the discovery of America, it is known with certainty that Christopher Columbus did read the Imago mundi and made notes on his copy of the book, but this was not until after he returned from his first voyage to America.

M. Partonnier de Gandillac, Usage et valeur des arguments probables chez Pirre d'Ailly, AHDLMA 8 (1933), 43-92; B. Meller, Studien zur Erkenntnislehre des Peter von Ailly, Fr 1954; G. Lindbeck, Nominalism and the Problem of Meaning as Illustrated by Pierre d'Ailly on Predestination and Justification, HThR 52 (1959), 43-60; F. Oakley, Pierre d'Ailly and the Absolute Power of God. Another Note on the Theology of Nominalism, HThR 56 (1963), 59-73; idem, The Political Thought of Peter of Ailly, the Voluntarist Tradition NH-Lo 1964; W. J. Courtenay, Covenant and Causality in Pierre d'Ailly, Speculum 46 (1971), 94-119; R. P. Desharnais, Reassessing Nominalism: A Note on the Epistemology and Metaphysics of Peter d'Ailly, Franciscan Studies 34 (1974), 296-305; L. Kaczmarek, Modi significandi und ihre Destrutruktionen. Zwei Texte zur scolastischen Sprachtheorie im 14. Jahrhundert, Mr 1980; L. B. Pascoe, Pierre d'Ailly: Histoire, schisme et l'antéchriste in Genèse et débuts du Grand Schisme d'Occident (1302-1394), P 1980, 615-622; A. Maierù, Logique et théologie trinitaire: Pierre d'Ailly in Preuve et raisons à l'Université de Paris, logique, ontologie et théologie au XIVe siècle, P 1984, 253-268; O. Pluta, Utinam haberem hoc! Sprachphilosophische Betrachtungen bei Adam Wodeham, Gregor von Rimini und Peter von Ailly in Rekonstruction und Interpretation, T 1985, 23-51; idem, Albert von Köln und Peter von Ailly, FzPhTh 32 (1985), 261-271; M. Chappuis, L. Kaczmarek, O. Pluta, Die philosophischen Schriften des Peter von Ailly, FzPhTh 33 (1986), 593-615; A. E. Bernstein, Pierre d'Ailly and the Blanchard Affair, Lei 1987; O. Pluta, Die philosophische Psychologie des Peter von Ailly, Ein Beitrag zur Geschichte der Philosophie des späten Mittelalters, A 1987; B. Buenée, Entre l'Église et l'État. Quatre vies de prélats français à la fin du Moyen Âge, P 1987, 125-299; L. Kaczmarek, "Notitia" bei Peter von Ailly, Sent. 1, q. 3. Anmerkungen zu Quellen und Textgestalt, in Die Philosophie im 14. und 15. Jahrhundert, A 1988, 384-420; J. Biard, Logique et théorie du signe au XIVe siècle, P 1989; M. Kaczmarek, Vitalis immutatio. Erkundungen zur erkenntnispsychologischen Terminologie der Spätscholastik in Mathesis rationis. Festschrift für H. Schepers, Mr 1990, 189-206; J. Biard, Présence et représentation chez Pierre d'Ailly. Quelques problèmes de théorie de la connaissance au XIVe siècle, Dialogue 31, (1992), 459-474; M. Chappuis, Le Traité de Pierre d'Ailly sur la Consolation de Boèce, Q. 1, A-Ph 1993; P. J. J. M. Bakker, Syncatégorèmes, concepts, équivocité. Deux questiones anonymes, conservées dans le ms. Paris, B.N., lat. 16.401, liées à la sémantique de Pierre d'Ailly, Vivarium 34 (1996), 76-131.

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