It was St. Thomas Aquinas who baptised Aristotle. Plato had more or less been a constant source of inspiration for the Christian thinkers from the very beginning, but Aristotle, with its appeal to senses and a description of the world based in how it appeared to man’s eyes, was a bigger challenge. The understanding of Aristotle’s philosophy under the light of Christianity had begun before St. Thomas. His mentor, St. Albert the Great was a worthy predecessor, but there is no doubt that as Aristotle was the systemiser of the Greek world and it was St. Thomas who achieved the same result for Christendom based on the ideas of the Stagirite. From St. Thomas onward Aristotle became The Philosopher in the Western World. The movement for nation states to impose themselves above the Church was also a revolt against Aristotle’s preeminence.
Both scholar’s purpose was to find the natural laws that conditioned man’s existence. Both derived these laws from human reason’s most powerful tool – logic (namely, the laws of Identity, No-Contradiction and The Excluded Middle) – the difference being that while Aristotle though these originated in man’s particular set of properties and attributes (and thus helped men in finding what was good for them), Saint Thomas believed, since Creation had been set in an intelligible way, that reason was a God given tool to help men deduct what was good for them. There was, of course, a Positive Law directly taken from Revelation, but this was just meant to be an easy path to know what is good. Positive Law could not contradict Natural Law. If it appeared to do so, then it was a question of misinterpretation of the former.
Another difference in their interpretation of Natural Law was its consequences for the individual. Aristotle came from the Greek Polis, the nuclear city-state entity to which all citizens had a duty to perform. Furthermore, being a part of the aristocratic elite, he did not found the need to derive individual natural rights from the postulates of Natural Law. He defended slavery as something natural. To Saint Thomas Aquinas, matters presented themselves differently. Despite also coming from aristocratic origin (it was another type of aristocracy, their fortune based on trade and commercial ventures unlike the landowner, agricultural background of Classical Greek aristocracy), Saint Thomas devoted his life to the Church where the idea of Salvation is the ultimate goal and the present responsibility of each and every individual. Thus, for Salvation to be conceivable, individuals endowed by the creator with free-will necessarily had to possess a set of rights naturally inherent to them. The consequence of Natural Law was thus, the existence of Natural Rights for individuals.
Despite such opposing starting points – one from the physical world, the other from the spiritual world – they were both moralists in their conclusions as both believed there was an ethical code to be derived from Natural Law since this Law, interpreted with Human Reason, allowed the identification of good, and hence, of the good way to act. Nonetheless, both Aristotle and St. Thomas, despite using their own reason, allowed certain moral beliefs they previously accepted as correct to affect their conclusions to specific issues in ways that contradict their own analysis. Such is the case for Saint Thomas when he condemns the charging of interest but at the same time praises the virtue of the Societas (where the effects on the use and property of money are the same) and Aristotle when he tries to find the just price for an interchange using mathematics despite concluding that the demand is set by the use-value individuals give to goods.
Both Aristotle and Saint Thomas Aquinas look into economic problems as a particular field of application of Natural Law. This precedes Ludwig von Mises findings that Economics is a subset of the much wider science of Human Action or praxeology. Although many of Saint Thomas findings follow the same arguments and line of reasoning of Aristotle, it is only natural to assume that their conclusions coincide many times because they were applying the same tool to the phenomena under study – aristotelic logic.
The most notorious difference between them is that Aristotle despised moneymaking and the activities of trading, transporting and hiring labour as unnatural needs that should be abandoned while Saint Thomas view them as beneficial, not only for the individual, but also for society as a whole, as merchants take goods from where they are abundant to where they are scarce. This is most likely due the different nature of their aristocratic origins as we saw above, Aristotle despised manual labour while Aquinas saw it as something valuable, Nonetheless, it can also be related to Aristotle’s understanding of the polis as the organic unit of society. Despite his disdain for individual trading and industry, he acknowledges that these functions are important for the polis and defends that these activities should be fostered between cities, as if this could be done without the intervention of individual human beings.
On the other hand their views on exchange, money and private property are very similar. Exchange is probably the most delicate aspect when bringing them together. Aristotle’s starting point is that the demand of a good lies in its use value and even goes as far as to understand the imputation argument for capital goods: If the imputation of A to valuable good C is more appreciated than the imputation of B to C, then A will be more valuable than B; and that things could be valued by their loss: if the loss of A produces more grief than the loss of B then A is more valuable. But then he went on to say that things are exchanged when there is equality in their value. This is not so. Things are exchanged when there is an inequality in their values, meaning, when each participant values more what the other has to offer than what he has to give in return. But the equality argument makes room for mathematical calculations as two sides of an equation allow weighting one against the other. This means a proportion between the values of goods could, in theory be found. Probably because Aristotle knows it is not so, he shifts his calculations from the goods themselves to the cost endured by the producers of those goods. So, instead of trying to find a fixed price or ratio of exchange between a house and a determined number of shoes, he makes use of mathematics to arrive at a ratio builder/shoemaker. This is most unfortunate as some historians subscribe the view that Aristotle was a supporter of Labour Theory of Value when in fact the whole explanation of that ratio is too obscure to allow for any such conclusions to be drawn. It could be that he was trying to tell that, if the price of a house when compared with the shoes produced for exchange is very profitable, then more people (resources) would translate from one sector to the other, as they discover this and some sort of long term equilibrium would apply.
Saint Thomas made what could be described as the opposite journey. His starting point must have been that of the Gratian code, against the turpe lucrum and taking into consideration the laesio enormis. Backed by the views of his mentor, St. Albert the Great, and the value-use argument of Aristotle, Aquinas came into consider the market price as the just price. This was not a straightforward path. Aristotle’s considerations towards objectivity in value could have influenced St. Thomas when he stated that the just price is derived from utility plus labour and expenses. This expression is also taken as an argument that St. Thomas believed the amount of labour determined the value of goods. This argument is a little over stretched as at no point Saint Thomas refers to labour alone and, more importantly, this definition (commonly used by merchants to justify their profits) is absent from his most mature work, the Summa Theologica. In fact, when describing merchants’ valuable activity of taking goods from where they are expendable to where they are necessary, Saint Thomas gives the example of a merchant that is the first to arrive with food to a hunger-stricken place. Knowing that there are others who will soon follow, Saint Thomas says that there is no obligation for this merchant to pass the information he possesses to his clients (even because there is no certainty as it is a future event) and still the price they voluntarily agree to pay for his goods is a just price. So, to Saint Thomas, as to Aristotle ,it is demand that determines the utility of a good.
When it comes to money their views practically coincide. They both identify its functions as a means of exchange, a unit of account and a store of value (Saint Thomas accurately observing that, nonetheless, the value of money changes – albeit to less extent – than that of other goods). Both of them condemn interest more from a moral point of view than from careful analysis. To Aristotle money is barren. That it produces more money just because it is lent should be considered unnatural. Yet the fact that people voluntarily agreed to such exchange should have made him consider it was for mutual benefit. He failed of course to appreciate that in a loan present goods are being exchanged for future goods and time preference naturally sets a price. Saint Thomas states that, as money is the measure of all goods, and is itself measured by its face value, its value cannot change in relation with itself. He also fails to acknowledge that money today and money in the future, seen from today, is not the same thing. But maybe he hinted this as he takes the argument a little further and says that money is consumed in the act of lending and its use is the same as its property. Thus, charging an interest is the same as charging twice, once for its use, the other for its property. Nonetheless he defends the commercial venture embodied in the Societas, as in this case, the lender does not lose the property of his money. Both Aristotle and Saint Thomas condemnation of usury derives more from their moral judgements than from careful analysis of the facts.
Lastly it should be mentioned that both defend private property on the grounds that it is a condition for a peaceful society (against what Plato believed) and that it is more productive as the owner will take better care of it than of a communal property (a proto description of the Tragedy of the Commons). Aristotle warns against the tabula rasa social experiments by stating that private property as been with mankind throughout history and that it allows men to act morally as benevolence and philanthropy can only come out of someone’s possessions. He also says that private property is implanted in man’s nature an argument that could also justify Saint Thomas defence of self-ownership.