to be a father, is not the same thing as to be a son.
 John Stuart Mill, A System Of Logic, Ratiocinative And Inductive (1843). copy citation

Context

“It is obvious, in fact, that if we take any two correlative names, father and son for instance, though the objects denoted by the names are different, they both, in a certain sense, connote the same thing. They can not, indeed, be said to connote the same attribute: to be a father, is not the same thing as to be a son. But when we call one man a father, another a son, what we mean to affirm is a set of facts, which are exactly the same in both cases. To predicate of A that he is the father of B, and of B that he is the son of A, is to assert one and the same fact in different words.” source