By our statement, “animal,” we exclude whatever is not an animal, such as inanimate objects.
By our statement, “rational,” we exclude what is not rational, such as brute beasts. The definition is therefore complete: nothing alien to the subject can enter, and nothing that belongs to it is left out.

Now, were we to say that a human being is merely “an animal,” this would be invalid. Why? Because it would admit what does not belong to the definition: if we say that man is an animal, both irrational beasts and rational creatures are included.
Likewise, if we said that man is “a rational, intelligent animal,” this too would be unsound, for it would exclude certain members of the class—namely the insane.

Hence a definition must always be both inclusive and exclusive.

In the same way, if we define wuḍūʾ (ritual ablution) as “washing the four limbs only,” this is unacceptable; one must add “in a specific manner,” because if you were to wash those limbs out of the prescribed order, it would not qualify as a valid wuḍūʾ.
And if you said, “Wuḍūʾ is washing the four limbs three times in a specific manner,” this too would be incorrect, for it would exclude a valid form of wuḍūʾ in which each limb is washed only once.

In sum, a ḥadd—i.e., a definition—is “a comprehensive description encompassing its subject and distinguishing it from everything else.”
Its requirement is that it be both inclusive and exclusive: no instance of the defined class falls outside it, and nothing foreign enters into it.