Letting contracts are those in which, their main effect is making permission. In this type of contracts, at the result of offer and acceptance, permission is obtained. Of the features of permission, is its dependence on the will of the contracting parties. Thus, in any case, that the will of person disappears, permission is void. Permission void, causes terminating legal operation. The study of brain anatomy shows that the cortex is the source of human will and in the cases that the activity of brain, named cortex, is stopped, the will disappears, the permission is missing, and the contract is terminated. Civil law, knows death and madness, and insanity, as the tools of letting contracts termination. But in other cases, the person in whom, his will is affected, and the content of consciousness is reduced, it is silent. The study in this case show that, when the activity of bran stops, or his consciousness reduces as madness, because of alcohol, drugs, psychotropic substances, the permission is missing, and the contract is terminated.