4. CUSTOMS must be _reasonable_[l]; or rather, taken negatively, they must not be unreasonable. Which is not always, as sir Edward Coke says[m], to be understood of every unlearned man's reason, but of artificial and legal reason, warranted by authority of law. Upon which account a custom may be good, though the particular reason of it cannot be assigned; for it sufficeth, if no good legal reason can be assigned against it. Thus a custom in a parish, that no man shall put his beasts into the common till the third of october, would be good; and yet it would be hard to shew the reason why that day in particular is fixed upon, rather than the day before or after. But a custom that no cattle shall be put in till the lord of the manor has first put in his, is unreasonable, and therefore bad: for peradventure the lord will never put in his; and then the tenants will lose all their profits[n]. [Footnote l: Litt. Sec. 212.] [Footnote m: 1 Inst. 62.] [Footnote n: Co. Copyh. Sec. 33.] 5. CUSTOMS ought to be _certain_. A custom, that lands shall descend to the most worthy of the owner's blood, is void; for how shall this worth be determined? But a custom to descend to the next male of the blood, exclusive of females, is certain, and therefore good[o]. A custom, to pay two pence an acre in lieu of tythes, is good; but to pay sometimes two pence and sometimes three pence, as the occupier of the land pleases, is bad for it's uncertainty. Yet a custom, to pay a year's improved value for a fine on a copyhold estate, is good: though the value is a thing uncertain. For the value may at any time be ascertained; and the maxim of law is, _id certum est, quod certum reddi potest_. [Footnote o: 1 Roll. Abr. 565.] 6. CUSTOMS, though established by consent, must be (when established) _compulsory_; and not left to the option of every man, whether he will use them or no. Therefore a custom, that all the inhabitants shall be rated toward the maintenance of a bridge, will be good; but a custom, that every man is to contribute thereto at his own pleasure, is idle and absurd, and, indeed, no custom at all. 7. LASTLY, customs must be _consistent_ with each other: one custom cannot be set up in opposition to another. For if both are really customs, then both are of equal antiquity, and both established by mutual consent: which to say of contradictory customs is absurd. Therefore, if one man prescribes that by custom he has a right to have windows looking into another's garden; the other cannot claim a right by custom to stop up or obstruct those windows: for these two contradictory customs cannot both be good, nor both stand together. He ought rather to deny the existence of the former custom[p]. [Footnote p: 9 Rep. 58.]