B. Corollary

9. From this follows a short corollary, that error or erroneous apprehension as to any condition per se required for a contract makes the contract null. And there are in general three conditions, two on the part of the persons: one, namely, when this person is thought to be that person, the other when an enslaved person is thought to be free. The third condition is the corresponding error on the part of the contract, as when one person were to believe that the other person was contracting with him in a like contract although, however, in the truth of the matter the other person is not contracting, because he does not intend to give as that person intends to receive.

10. As to the first error, it is plain that it impedes all buying and selling of things, as that if gold is bought and the seller is selling brass. So here, if someone intend to give to b power over his body and he give it to a, nothing from such a comes to be.

11. The second condition is a special one here, because here there is a substitution of power over a body for power over a body; but a slave cannot give power over his body to a free woman, as she conversely can; and she, not knowing his servitude and consequently not consenting knowingly to a slave, does not want to exchange power over her body for a power over a body that the slave does not have. And from this follows something else, that error of a better condition is not an impediment - and servitude likewise is not an impediment, if it is not unknown.

12. The third condition is general to every contract, because if one of the two is contracting with a straight mind and the other is only using signs of a contract without a mind to contract, there is no contract; because the more principal cause is lacking, namely the mutual act of will, and the less principal one that is in place, namely the extrinsic sign, does not suffice.

13. This contract [of matrimony] therefore, when it limps on one side, is dissolved; for a contract of matrimony cannot limp since there is a transfer of equivalence there -just as there is a likeness between things that are alike, for one thing cannot be like another thing if the other is not like it.