Summa summae S. Thomae, Tomus III

You will ask first what possession is.

I answer. Possession is said from the position of the feet, because over immovables it is usually acquired by the position of feet. It is commonly defined: "Detention of a thing with the aid of body, mind, and right."

It is explained. It is said, "detention:" under which name is understood also the first apprehension, which is the beginning and acquisition of possession.

It is said, "of a thing," not only corporeal, as a field, a house, or movables, but also incorporeal, as an annuity, a benefice, a servitude, and similar rights, which are said to be possessed in a manner.

It is said, "with the aid of the body;" because it does not begin, nor is it possession, unless there comes an external or corporeal act by which the thing is apprehended by foot, hand, eyes, or some similar way, in itself or in another in which it is held to be contained, as I shall say presently.

It is said, "with the aid of the mind;" because the intention of possessing and holding the thing for oneself and not for another is required. Hence a depositary does not properly possess the deposit, nor a tutor the goods of a minor.

It is said, "with the aid of right," not that it is necessary that right approve it, otherwise every possession would be just, but because for possession it is required that right not impede the detention of a thing from being held its possession. For this reason, detention of a sacred thing by a layman is not possession, not even of fact, since right prohibits it; but detention of a stolen profane thing, though unjust, is nevertheless true possession of fact, because right does not impede the detention of such a thing from sufficing for possession.

You will ask secondly in how many ways possession is twofold.

I answer. Among the various divisions of possession (for one is just, another unjust, one in one's own name, another in another's name), the principal is into civil, natural, and most civil.

Civil is that which, begun with the aid of the body, is retained and continued by the aid of the mind alone. Thus he possesses civilly who is absent from his things, but holds and occupies them as his own in mind and faculty, so long as he is not cast out of possession by another. It is called civil, because it rests more on civil than on natural law, since the natural mode of possessing is by bodily insistence, and also because it produces a civil effect, namely prescription.

Natural is that by which a thing is occupied by body and mind together. But one is held to occupy corporally when he is so near the thing that he can be present to it immediately. This natural possession can be separated from civil: for a thief, e. g., possesses my thing naturally, I civilly, until by abstraction of it from place to place it is put outside my custody; for then I am cast out of civil possession.

Most civil is that which is acquired by the sole disposition of law, even by one who knows nothing of it, and therefore without the aid of body and mind. Thus the infant heir possesses the inheritance of the deceased, especially in France and the country of Liege, where the dead clothes the living: Le mort saisit le vif. The reason is, because the mode of acquiring possession is subject to human law, which can establish diverse modes according as it seems more expedient for the commonwealth.

You will ask thirdly whether, possession lost, dominion is always lost.

I answer negatively. For of a thing lost or taken by theft I lose possession, yet not dominion.

You will ask fourthly what are the privileges of possession.

I answer. The chief privileges of possession are: first, that it produces prescription; second, that in doubt the condition of the possessor is better; third, that it is not for the possessor to prove that the thing is his, but for the assailant to prove that it is not. Concerning possession see more among the interpreters of the laws.