There used to be many categories a thing could belong to in the Roman law. The notion of thing itself was seen as very comprehensive and it contained both material things and various abstract concepts. A short description of the categories can help explain the extent of this notion and diversity of things in ancient Rome.
In ancient Rome the concept of things encompassed objects, cases, circumstances, events, actions, subjects of disputes as well as processes. One of the basic criteria of division of things was the division into tangible and intangible things – res corporales and res incorporales. This division originated from Greek philosophy. As explained by Gaius, the things belonging to the first category could be studied with the use of senses (land, slave, robe, gold), while things from the second category could not – for example inheritance, usufruct or obligation.
Another line of division was the division into things that could be subject to the rights of private persons and things excluded from circulation – res in patrimonio and res extra patrimonium. Furthermore, things excluded from legal circulation were further divided into things excluded on the basis of human law (res humani iuris) and things excluded on the basis of divine law (res divini iuris). In the first category there were things common for all people, res omnium communes, intended for everyone’s use, like air, flowing water, sea, as well as res publicae and res universitatis, which were public things belonging to the state and private owners, respectively, for example harbours, roads, bridges, stadiums, theatres, baths, squares and statues. Things excluded from circulation on the basis of divine law included res sacrae – things ceremonially devoted to deities, especially temples and altars, res religiosae – connected to the worship of the dead, especially tombs, and res sanctae – put under the protection of the gods without the act of devotion, like city walls, gates, border signs.
Another division was the division based on the method of acquisition. Res mancipii required the use of mancipation or in iure cessio, and they encompassed all the lands in Italy, the oldest rural land easements and pack and draft animals. It is worth noting that only oxen, horses, donkeys and mules fell in that category. Elephants and camels discovered by the Romans at a later time belonged together with other lands to the category of res nec mancipii and tradition was sufficient to transfer their ownership. This division was formally abolished in the times of Justinian.
The division into movable and immovable things was introduced already in the Law of the Twelve Tables. Res immobiles were defined as marked ground together with all things connected to it, like plants (natural connection), buildings (artificial connection) and minerals (understood as everything found underground). Contrary to res mobiles, defined as everything that is not immovable, it was not possible to change the location of immovable things. Moreover, only movable things could be subject to theft.
An important division was the division into singular, complex and collective things. Things in the first category were indivisible, they constituted one whole, for example a stone, an animal, a slave (natural origin), an amphora, a painting (artificial origin). Legal relations concerned them as a whole. Complex things were made of singular things permanently connected into a new thing, like a building or a ship. In the moment of creation of a complex thing, the legal relations concerning its component things were suspended, the components lost the character of separate entities, however in some circumstances it was possible for an owner of a component thing to request its detachment, if it was possible. Collective things were collections of singular or complex things having a single economic purpose and a common name, for example a herd, a library. Such a collective thing remained the same thing even after its individual pieces were replaced, while legal relations could concern both the thing as a whole and its individual parts.
Things were also divided into things determined by their type and things determined by their individual features. The first kind (genus) encompassed things like grain, oil, money. They were interchangeable, they could be weighted, measured, counted. The second kind (species) had specific individual features, like a specific horse, slave or sculpture, they were not substitutable.
A separate division was the division into consumable and non-consumable things. The use of consumable things lead to their wear; wine, grain, oil and fuel belonged to this category. Non-consumable things were intended for repeated use – land, furniture, tools, slaves, animals.
The last criterion was the divisibility of things. Indivisible things could not be divided into pieces without losing their properties and value. It concerned animals, slaves, pieces of art. Divisible things could be divided freely without any change to their value.
There were also separate categories of things, like attachments and utilities. Attachments were things separate from the main thing which did not belong to its permanent pieces and were intended for improving the economic utility of the main thing, like shutters on a building. Utilities (fructus) were the fruits which were generated by a given thing without changing or destroying the main thing, they could be natural (fruits, vegetables, milk, eggs, wool) or civil (rent).