An easy but efficient way to understand the meaning of common good is to compare it to the concept of global good. The global good can be instead metaphorically rendered with the image of a summation, whose addends well represent the individual goods (or the goods of social groups that makes society), the common good is rather comparable to a multiplication, whose factors represent the goods of individuals (or groups).

The metaphor is immediate: if even some of the addenda is cancelled in a summatory, the total sum remains anyway positive. In fact, it can also happen that if the objective is to maximise the total good (for example the national PIL) it’s convenient to “cancel” the good (or wellbeing) of someone as long as someone else’s profit of wealth increases sufficiently and does more than compensating it. It’s not like this on the other hand with a multiplication: the cancellation of only even one element resets the entire product.

In other words, the logic of the common good doesn’t admit trade-off: you cannot sacrifice the good of someone- no matter his/her life situation or social setup- to improve the good of someone else and this for the essential reason that that person is still a human being! For the logic of the total good, instead, that someone is an individual, that means an individual identified by a particular utility function and utilities- as we know- can easily be summed or (be compared), because they have no face which means no identity nor history. Who’s the “enemy” of the common good? On one hand, those who behave like “free riders”, that is those who live off of others.

On the other hand those who act as pure altruists, that is those who nullify their self-interest in order to foster the interest of others. Both behaviours don’t feed the common good, if only for various reasons and different consequences. Neither pure selfishness nor pure altruism are able to make sustainable -on their own- a human social order. Who’s the “friend” of the common good? A behaviour inspired by the principle of reciprocity.

The principle of reciprocity sounds like this: "I give you freely something so that you can give in return according to your capabilities, to others or eventually to me” .
On the contrary, the exchange principle says: "I give you something as long as you give me something of equivalent value in return”.
The principle of reciprocity assumes the proportionality, the exchange principle assumes the equivalency.