Sentence:  According to him, God cannot be called the Highest Value, because there in no <WSD>'unvalue</WSD>' with which he can be contrasted. But, here Alexander confuses the absolute predicate with relative ones. If a thing has value, it is in contrast to some <WSD>unvalue</WSD>, since all predicts affirmed about finite things are relative. But, applied to God, the case is otherwise. All that is said about God is absolutely predicated. 
Meaning of the word marked with <WSD> tags:  A negative or undesirable value (all senses)