For you have to show either that it is not true at all to apply the account to that to which the name is applied (for the definition of man ought to be true of every man); or that though the object has a genus, he has failed to put the object defined into the genus, or to put it into the appropriate genus (for the framer of a definition should first place the object in its genus, and then append its differences; for of the elements of the definition the genus seems to be the principal mark of the substance of what is defined); or that the account is not proper to the object (for, as we said above as well, a definition ought to be proper); or else see if, though he has observed all the aforesaid cautions, he has yet failed to define the object, that is, to express the essence of what is being defined.