5 Another circumstance by which a stipulation may be avoided is want of correspondence between question and answer, as where a man stipulates from you for payment of ten aurei, and you promise five, or vice versa; or where his question is unconditional, your answer conditional, or vice versa, provided only that in this latter case the difference is express and clear; that is to say, if he stipulates for payment on fulfilment of a condition, or on some determinate future day, and you answer: 'I. promise to pay today,' the contract is void; but if you merely answer: 'I promise,' you are held by this laconic reply to have undertaken payment on the day, or subject to the condition specified; for it is not essential that every word used by the stipulator should be repeated in the answer of the promise. 6 Again, no valid stipulation can be made between two persons of whom one is in the power of the other. A slave indeed cannot be under an obligation to either his master or anybody else: but children in power can be bound in favour of any one except their own paterfamilias. 7 The dumb, of course, cannot either stipulate or promise, nor can the deaf, for the promisee in stipulation must hear the answer, and the promisor must hear the question; and this makes it clear that we are speaking of persons only who are stone deaf, not of those who (as it is said) are hard of hearing. 8 A lunatic cannot enter into any contract at all, because he does not understand what he is doing. 9 On the other hand a pupil can enter into any contract, provided that he has his guardian's authority, when necessary, as it is for incurring an obligation, though not for imposing an obligation on another person. 10 This concession of legal capacity of disposition is manifestly reasonable in respect of children who have acquired to some understanding, for children below the age of seven years, or who have just passed that age, resemble lunatics in want of intelligence. Those, however, who have just completed their seventh year are permitted, by a beneficent interpretation of the law, in order to promote their interests, to have the same capacity as those approaching the age of puberty; but a child below the latter age, who is in paternal power, cannot bind himself even with his father's sanction. 11 An impossible condition is one which, according to the course of nature, cannot be fulfilled, as, for instance, if one says: 'Do you promise to give if I. touch the sky with my finger?' But if the stipulation runs: 'Do you promise to give if I do not touch the sky with my finger?' it is considered unconditional, and accordingly can be sued upon at once.