Heytesbury sets the following conditions which must be obtained for limits to exist: (1) there must be a range in which the capacity can act or be acted upon, and another range in which it cannot act or be acted upon, and not both; (2) each capacity should only be able to take a value in the range on which it is measured from zero and the value which serves as its boundary. Thus, if an active capacity is capable of acting upon a given passive capacity in the range, it must be capable of acting u