Abstract
The notion of iustum pretium - the " juste prix " - is both a central problem of antiquity and a thread running through the reception of Roman law in medieval Europe. The last three decades have revived interest in pre-modern economic thought and its religious matrix, notably Franciscan. The work of Pierre de Jean Olivi, which is currently being rediscovered, is a prime example of this : his Tractatus de contractibus (late in the XIIIth century) questions the morality of commercial acts. This questioning, which accompanied urban growth and the rise of the mercantile economy, was rooted in a legal culture that drew heavily on the Corpus Iuris Civilis, rediscovered as early as the 11th century, which had become the basis of lectures for civil and canon lawyers. However, a methodological limitation persists : historiography often tends to consider Roman law only as it was expressly retained by the medievalists, whereas for the latter, the Corpus was a living whole, consulted in its entirety.
In the Middle Ages, two closely related issues polarized thinking: usury and the purchase/sale price. Thomas Aquinas articulated them explicitly(STh. IIª-IIae, q. 77, pr.), contrasting fraud in exchange and usury in lending. The condemnation of usury is based on a theological logic : to demand more than one has given is to enrich oneself unjustly at the expense of others and, more radically, to " steal " time, which belongs to God ; thus the Decree of Gratian (II, 14, 3, c. 1). But if usury is a sin, the right price is more a matter of contractual fairness. For Thomas, the Aristotelian background is decisive : justice(dikaiosynē) is opposed to pleonexia, greed. Selling " too expensive " and lending at interest thus become forms of undue capture of advantage. In the Roman world, the Aristotelian imprint may be present, but the angle is first and foremost strictly legal.
To understand Roman iustum pretium, we need to look at the context in which it was expressed. Two substantives structure : pretium (result/value of exchange) and aestimatio (valuation operation), often qualified by iustum or verum. Above all, the question of " fair price " very frequently arises outside the context of sales : it is first and foremost a judicial one. In the event of a conviction, a Roman lawsuit results in the award of a sum of money : the judge must determine its value. Thus, if a will imposes a bequest on a thing belonging to another and sale is impossible or extravagant, the heir is released by iusta aestimatio (D. 32.14.2 Gai. 1 fideicomm.). In the action for partition, the judge appraises at the fair price (D. 10.3.10.2 Paul, Book XXIII On the edict). In cases of theft, the penalty is calculated on the basis of verum pretium, notid quod interest (D. 47.2.50 Ulpian, Book XXXVII On the Edict). The Falcidia law (40 BC) - reserving at least a quarter of the estate for the heir - also assumes a valuation " according to the truth " (D. 35.2.42 Ulpian, book XIV On the edict). The Aquilia law (286 B.C.) posed the problem of estimating damage and inspired a formula that was to last, reported by Paul : the value of things is not determined ex affectione nec utilitate singulorum, but " communément " (D. 9.2.33 Paul, Book II On Plautius), i.e. according to an objective measure, widely shared, and freed from particular attachments and singular utility.
This objectification brings us back to the ordinary experience of exchanges : to deny the existence of a price " common " would be to contradict the obvious. In practice, the price cannot be reduced to the arbitrary agreement of two individuals unaware of the market value ; it results from the bundle of offers and demands, from repeated comparisons and arbitrations, producing a current average. It is this median that jurists refer to as iustum pretium or verum pretium. Paul insists : prices vary according to place and time - oil is not valued in the same way in Rome and Hispania, neither in dearth nor in abundance (D. 35.2.63.2 Paul, book II Sur la loi Iulia et Papia) - ; Gaius extends the observation to wine, oil, wheat and even money, whose availability and rates differ according to region (D. 13.4.3). The Romans also had a precise economic lexicon : annona (annual production, then price of commodities) and caritas annonae (dearness through scarcity) testify to a clear awareness of the link between scarcity and price ; conversatio publica designates the circulation of exchanges, i.e. an abstract figure of the " market ". For example, a law passed by Julian (Theodosian Code, 14.3.2, year 362/363) ordered that the tax on pork in Campania should be calculated according to the prices observed in the local conversatio publica, and not according to those in Rome : the market has its own territorial size and temporality, which the assessment must respect.
From this perspective, the Roman judge does not invent the value : he establishes it and sets it as an impartial third party. Judicialaestimatio aims for a point of equilibrium - an average value acceptable to the parties - and is based on the observation of actual exchanges. Hence the insistence on the communiter fungi, too often misunderstood by modern historiography : it does not refer to a normative community control of prices, but to the current price as it results from free and repeated transactions in a given place and time. Because Roman markets formed neither a unified " supermarket " nor a homogeneous space for monetary circulation, but a constellation of interdependent local and interregional circuits, value is fundamentally contextual ; it can vary greatly from one terroir to another, from one season to another, according to supply, demand and the bargaining power of the co-contractors. This does not abolish the iustum pretium ; rather, it clarifies the conditions for determining it.
Understood in this way, the Roman notion of a fair price is not a moral speculation based on exchange : it is an operative category of law, mobilized in relation to litigation, inheritance, Aquilian liability and, of course, sale. Its medieval reception inserted it into a theological-moral horizon - sin, equity, Aristotelianism - without exhausting its legal scope. For the historian, the challenge is twofold : to restore Roman thought to its proper logic (where pretium is an exchange value andaestimatio an objectification operation carried out by the judge) and, on this basis, to measure the originality of medieval elaborations. In the end, the fair price remains the " average " and " true " - iustum/verum pretium - that the publica conversatio generates through the interplay of exchanges. It is because this notion soberly articulates law, economics and society that it has survived the centuries and still retains its singular explanatory power today.
