Summa summae S. Thomae, Tomus III

[By the name of accession is understood the right by which the owner of the principal thing becomes owner of the accessory thing, that is, of all things produced from some thing, or naturally or artificially united to it.

Accession is either natural, or industrial, or mixed.

I. — Natural accession occurs : 1° By birth, namely when from your animal another is born, conceived indeed of a male belonging to another. For the axiom holds : « The offspring follows the womb. » — 2° By fructification, that is, the production of fruits from land without human industry. — 3° By alluvion, when by the force of waters an increment gradually accedes to your field. But if a notable part of someone's land suddenly accedes to another's field by avulsion, the former owner of it does not lose dominion if he exercises his right within the useful time¹.

II. — Industrial or artificial accession can occur : 1° By building, when one builds on his own land with another's materials, or on another's land with his own materials. Then in general the building is his whose the land is, due compensation being made to him who built in good faith². Yet on this matter the laws of each region are to be consulted³

• 2° By specification, when to pre-existing matter a new form or species accedes, v. g., if from wool belonging to another you have made a garment, or from another's wood or gold a vessel. By common law, the new species accrues to the artisan, the price of the material being paid, if it cannot be reduced to its prior form, and in conscience probable good faith is required : otherwise the thing accrues to the owner of the material, v. g., a golden vase⁴. Under the new laws, in either case the thing accrues to the owner of the material, unless the work of the artisan far exceeds the material in price⁵.

— 3° By adjunction, when someone's thing is so joined to another's thing that together they make one whole ; in which case the whole composite pertains to him whose thing is considered as something principal, who however must make the owner of the accessory part whole. So a usufructuary who has painted the walls of a house cannot erase the painting with damage to the walls. Conversely, a painter who has expressed an outstanding image on another's canvas can keep the painted canvas, its price being paid ; for, as Justinian says, Instit. book II, tit. 1, n. 34, « it is ridiculous that the painting of Apelles should pass as an accession to the cheapest board. » So by the law of almost all peoples⁶

— 4° By commixture, when solid things, and by confusion, when liquid things belonging to different persons are joined. By Roman law, if commixture or confusion has been made of several things of different genus, from them a new species arises, which falls under the principle of specification expounded above. But if no new species has been made, and the commixed or confused things can be separated without becoming worse, each must be separated for its own owner. Otherwise, each will have a right in the whole mass, according to each one's portion or price⁷. According to the new laws, the thing becomes common, and must be sold under the hammer, and the price divided, unless it has been otherwise stipulated among the owners.

But if one part is principal, and much exceeds the others in price or quantity, it is licit for its owner to vindicate the whole to himself, a just price being refunded to the others⁸.

III. — Mixed accession occurs especially by planting and sowing. It is called « mixed, » because nature and industry concur in this. It is subjected to the same rules⁹ as building¹⁰.