where a person has built on his own land with another's materials; he is taken to  be the owner of the building because everything built into it cedes to the land. He  who was owner of the materials does not cease to own them, but he cannot recover  what is his own; he will obtain their double value. If the building is destroyed he  may recover his property, unless he has secured the double value. Conversely, if  one in bad faith builds with his own materials upon another's land, he is presumed  to have given him the materials; if in good faith, let the owner of the land pay him  the price of the materials and the wages of the workmen. The above is true where  the building is immovable; if it is mobile the rule will be otherwise, as where a  barn for the new wheat, made of wooden planks, is placed on Sempronius's estate;  it will not become his property. This kind of accession produced by the intervention  of human action may be seen in connection with letters. For writing, though in  letters of gold, becomes part of parchments and charters exactly as things built  upon land or crops sown become part of the soil. But a contrary rule will apply to  paintings. It would be ridiculous if by accession a valuable painting became part  of a worthless panel. Thus the panel cedes to the painting, [as explained more  fully in the Institutes,15 and it may there be found [and] in the Summa that in  some cases the position of the possessor is the weaker.16 Also in the same that his  position is the stronger because of the two-fold advantage of possessing.17 Also  in the same, that where the rights of each are obscure, judgment will be given for  the possessor.] The same sort of accession may be seen in connection with textiles,  for if one has woven the purple thread of another into his vestment, though the  purple is the more valuable it nevertheless becomes part of the vestment by  [right] of accession. The same kind of accession may be seen in connection with  usufructuaries and usuaries of land, in connection with gaining the fruits.18
Of accession made by natural and human forces jointly.
 With respect to them 19the accession by which dominion is acquired will be a  combination of natural and human action, as where Titius places another man's  shrub in land belonging to himself, the shrub will be his. And conversely, if Titius  plants his own shrub in soil belonging to Maevius, it will belong to Maevius, provided  in each case that the shrub takes root, for until it does so it remains the property of  its original owner. And it is also true that if a neighbour's tree so encroaches on the  land of Titius that it strikes its root into his soil, the tree will belong to Titius.  Reason does not permit it to belong to any other than to him in whose land it is  rooted. So if a tree is placed on the boundary of two estates and strikes root into a  neighbour's land, it will be the common property of the two landowners, nor will the  neighbour be permitted to cut off the roots. This