In this context, I disagree.
Its really clear what the good in "Common" means.
Its like principles of what the Constitution was written on. Life, Liberty and the pursuit of happiness.
Principles like these are qualities we all naturally exhibit and as such doesn't need any explanation or qualifications. I know in our modern world people have decided that everything is open to interpretation. But I believe this was indoctrinated into us, over a long period of time, not to consider the human value[0] aspect of our being that we are born with. This was done by watering down the direct meaning of words and slowly changing the perception that humanity doesn't have infinite value and has to rely on external power structures to give it meaning.
The problem with common good is that it's hard to agree upon what is good, and if you find a good, there's often a group that's outside this "common".