Is the Fourth Way really as simple as it looks? There are actual things better and worse. Therefore there is something actually best- a highest actual good.
Objection: a possible good could be higher.
Response: It is not higher as possible. More simply, actual things are better than possible ones. More importantly, we are only considering actual, existing goods.
Objection: There is no reason why more and less are said with respect to some most.
Response: The proof only requires that this be verified of things like good, true, dignified, etc. We will take goodness, since truth and dignity can be seen as kinds of goods.
If more and less good are not said with respect to some most, then more and less good are nothing other than relatives. But if relations change, it is because of the change of something other than the relation. But things can become more or less good. Therefore more and less good is said with respect to some most.
If all goods are relative to a higher and lower, then they all are essentially midpoints. But good is essentially an end or goal.
The simple answer is more plain: “wholly relative” is a contradiction- unless the relations are utterly changeless (Thomists and Augustinians wink at each other now).
Objection: There could be many highest actual goods.
Response: Right. It is irrational to expect the proof for the existence of something to tell you how many there are. Why does everyone assume that theology ends with existence? Theistic proofs prove something divine exists, and “highest good” is a divine thing. Give the theologian some time.