Spanish theologian
Martín de Azpilcueta (Azpilkueta in Basque) (13 December 1492 – 1 June 1586), or Doctor Navarrus, was a Navarrese canonist, theologian and economist.
Renowned in his time, he was the first to formulate the quantity theory of money in 1556 and was with Francisco de Vitoria and Domingo de Soto one of the main thinkers of the School of Salamanca.
Life.
Born in Barásoain in a noble family of Navarre, Martín de Azpilcueta was a relative of Francis Xavier. In 1508, he was amongst the rare students accepted in the new Colegio Mayor de San Ildefonso, founded by the cardinal Cisneros.
After his obtention of degrees in philosophy and theology at Alcalá, then for polical reasons he fleed in France in 1516 to study at the university of Toulouse and obtained a degree of doctor in civil and canon law in 1518.
Beginning in 1524, Azpilcueta served in several canon law chairs at the University of Salamanca. He got the chair of "Decretum" in 1532 and then obtained the first chair of canon law in 1537, when he taught to Diego de Covarrubias and Arias Piñel.
At the invitation of Charles V of Spain and John III of Portugal, he taught at Coimbra University in Portugal from 1538 to 1555. There he became friendly with Henry, future king of Portugal.
Back to Spain in 1555, he was charged by Philip II of Spain to defend his friend Bartolomé Carranza, Archbishop of Toledo, accused before the Tribunal of the Inquisition. At the age of eighty, he went to Rome to followed the process. Though he failed to exculpate the Archbishop, Azpilcueta was highly honoured at Rome by several popes, and was looked on as an oracle of learning and prudence. His humility, disinterestedness, and charity were proverbial. the pope Pius V made him counsellor of the Apostolic Penitentiary and tried, without success, to make him a cardinal.
Azpilcueta died in Rome at the age of 94. He is buried in the national Church of San Antonio de' Portoghesi. Among other lives of Azpilcueta there is one by his nephew, prefixed to the Roman edition of his works.
Works.
Economy.
In this work, Azpilcueta argued that the use of "money in exchanges is not unnatural," as Aristotle had claimed, and "put money on the same level as any other merchandise, and, consequently, established that the morality of exchanges did not depend on money as their object but on an equitable exchange". Then he legitimated the practice of interest against the catholic prohibition of usury.
Congitating about the effects of gold and silver arrivals from the Spanish Empire, Azpilcueta independently formulated the quantity theory of money in 1556. He also allegedly invented the mathematical concept of the time value of money.
In his work on the revenues of benefices, dedicated to Philip II of Spain and Pope Pius V, he maintained that beneficed clergymen were free to expend the fruits of their benefices only for their own necessary support and that of the poor.
Law.
His "Manual de confesores y penitentes" (1549), originally written in Spanish, was enormously influential in the fields of canon law and ethics, and by the first quarter of the seventeenth century, it had gone through 81 editions. The "Manual" made an important step in the development of moral theology as its own discipline. One of the four appendices Azpilcueta wrote for the "Manual", addressing exchange, supply and demand, and money, has recently been translated into English and published as "On Exchange" (2014). 
He formulated a basic form of the principle of contractual consensualim, which was later elaborated upon by other members of the School of Salamanca like Leonardus Lessius and Pedro de Oñate. 
Azpilcueta cogitated also on the concepts of commutative justice, just price and fairness in exchange: with a large interpretation of the principe of laesio and of the seventh Commandment, he considered that contracts doesn't be burdens for a party and that any violation of the equity can be take to ecclesiastical courts.
Use of vernacular language.
In the face of use by the reformed Church of natural languages in their liturgy and divulgation, the Catholic Counter-Reformation reacted by hanging onto Latin. On the other hand, Azpilcueta supported the use of vernaculars in his 1545 "Commento en Romance" released in Coimbra, writing down the prayers Our Father, Hail Mary, and Creed both in Latin and Romance, i.e. Castilian, coming up against the opposition of the defenders of the Latin tradition. 
Later, he had to explain in his 1586 "Miscellaneum centum" that vernaculars had been used before, as approved by bishops and inquisitors, citing "a pious and knowledgeable Cantabrian", referring to Sancho de Elso from Estella, who had used Basque in different prayers. He partially put down the unfamiliarity of "the rustic dwellers and highlanders" with the Christian teachings to the use of Latin, "instead of their native language, learning them by heart". 
References.
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