I'm trying to make sense of these 1,000-year old arguments (about the unity of God):
THE FIFTH ARGUMENT
The fifth argument, from the concepts of plurality and unity as follows.
In his Geometry (Book 5), Euclid defined unity as: "Unity is that property through which we say of any thing that is one". This means that by nature, unity precedes the individual thing, just as we say that heat precedes a hot object. If there were no "unity", we could not say of anything that it is one.
The idea which we need to form in our mind of unity is of oneness that is complete, a uniqueness, that is absolutely devoid of composition or resemblance. Free, in every respect of plurality or number, that is neither associated with anything nor dissociated from anything.
The idea of plurality is that of a sum of unities. Plurality therefore cannot precede unity of which it has been formed. If we conceive something plural with our intellect or perceive it through our senses, we will know with certainty that unity preceded it, just like when counting things, the number one precedes the rest of the numbers. Whoever thinks the Creator is more than one, must therefore nevertheless concede that there was a preceding unity, just as the numeral one precedes the other numbers, and just like the notion of unity precedes that of plurality.
THE SIXTH ARGUMENT
The sixth argument, from the incidental properties that attach to everything that is plural. Plurality is an incidental property ascribed to the essence, and comes under the category of quantity. Since He is the Creator of essence and incident, none of these attributes can be ascribed to His Being. For, it having been clearly demonstrated through scripture and reason that the Creator is above and beyond all comparison with, and similarity to, any of His creations, and seeing that plurality which adheres to the essence of anything that is plural is an incidental property - this property cannot be fittingly ascribed to the Creator's glorious Essence. And if He cannot be described as plural, He must certainly be One because there is nothing in between the two possibilities.
They seem to be based on neo-Platonic understandings about the relationship between the one and the many, but are not based on the Enneads of Plotinus
From al-Hidaya ila Fara'id al-Qulub c. 1080CE (while people still thought that both Plato & Aristotle were neo-Platonist in thinking).