Here is my paraphrase of the argument in ch.18:
It might seem that supreme goodness can exist where one person supremely loves and receives nothing in return from the other person for full happiness. But in fact such supreme goodness can’t even exist where only two persons mutually love each other. Suppose that, in divinity, there are only two persons. Then each gives and receives love, and each gives and receives the pleasure that such love brings. If each is alone, neither receives such love nor such pleasure. So supreme generosity requires three persons. If, in divinity, there are only two, neither shares such pleasure. But each divine person, being perfect, is supremely generous. Therefore, supreme goodness requires that if there are at least two divine persons, there are at least three persons.
Note that the first sentence seems out of place and does no work here. Really the argument here only begins with the third sentence. The only new thing here is the mention of supreme generosity. Supreme generosity requires that each of two divine persons have a third divine person with whom to share love and the pleausre such love brings. Not so to share would be less than supremely generous. But I don’t see that we really have a new argument here for at least three divine persons (if God exists). So that’s ch.18. Next up will be ch.19, which will be my final post for the series.
