Argument from degree
As formulated by Thomas Aquinas, the argument from degree is stated as follows:
"We notice that things in the world differ. There are degrees of, say, goodness or perfection. But we judge these degrees only by comparison with a maximum. Humans can be both good and bad, so the maximum goodness cannot rest in us. Therefore there must be some other maximum to set the standard for perfection, and we call that maximum God."
"That's an argument? You might as well say, people vary in smelliness but we can make the comparison only by reference to a perfect maximum of conceivable smelliness. Therefore there must exist a pre-eminently peerless stinker, and we call him God. Or substitute any dimension of comparison you like, and derive an equally fatuous conclusion."
- — Richard Dawkins, The God Delusion, 2006, p. 79
The Argument From Goodness
The argument from goodness is a special case of the argument from degree. In his Summa Theologica, Aquinas writes:
"The fourth way is taken from the gradation to be found in things. Among beings there are some more and some less good, true, noble and the like. But "more" and "less" are predicated of different things, according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest; so that there is something which is truest, something best, something noblest and, consequently, something which is uttermost being; for those things that are greatest in truth are greatest in being, as it is written in Metaph. ii. Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum heat, is the cause of all hot things. Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God."
- — Thomas Aquinas, Summa Theologica, question 1, article 3, objection 2
By saying, "as fire, which is the maximum heat", Aquinas scuttles his own argument: fusion reactions, which power stars (including the sun) are much hotter than fire. Aquinas, of course, did not know this, and therefore could not have based his idea of heat on the "maximum heat". Yet we do not doubt that he understood what "heat" means.
Likewise, we can know what "goodness" and "perfection" (in Aquinas's sense) mean by comparing people and actions to each other, and not to some superlative standard of goodness and perfection.
Secondly, a perfect standard can be useful without having concrete existence. For instance, we can mathematically define a perfect circle and use that as the standard for roundness. However, it is impossible to draw a perfect circle, and thus a perfect circle cannot exist. Similarly, the fact that we can imagine a perfectly good being in no way implies that that being actually exists.
Also, there's no reason the argument can't be applied with evil instead of (arbitrarily) good: "there is evil in the world, and the fact that we can perceive different standards of evil means there must be some maximal exponent of evil, and we call this supreme evil God."