Most would probably agree that if everyone had basic tangible needs, that would be a "common good." Most would casually agree that there is some sort of hierarchy of needs, as well, with food and shelter being near the bottom of that hierarchy, and cognitive and spiritual needs being near the top of the hierarchy, and that it is difficult to meet the higher order cognitive and spiritual needs during the uncontrolled weight loss of starvation. First things first. Not only is starvation bad for the individual, it's bad for the social order, because starvation is a real motivator that inspires risk-seeking counter-regulatory responses regardless of the perceived etiquette of making those responses. It is common sense that meeting basic needs is fundamental to the common good. I would also argue that maximizing higher needs also promotes the common good, but that's for a later post. Let's talk about more tangible things we can agree on first in the context of maximizing the common good.
Let's first consider some tangibles, such as food, housing, education, medical coverage, leisure, and so on, in terms of utility. In other words, what is the usefulness of food, housing, medical coverage, an education, leisure? Obviously, the usefulness of each depends on how much you already have. One boiled egg per day may delay death, but your chances of living improve a lot as you approach six eggs per day. However, at some point, simply adding eggs to your diet won't add any value to the diet and you chances of survival. Likewise, 9 square feet of housing may improve your survival, but 500 square feet is much more valuable. Adding second full housing unit does not add nearly as much value as the first. You need minimum amounts of a tangible before it's useful, but your ability to extract use saturates at some point. Let's simplify by representing these specific tangibles by money. If we have limited tangibles (or money) in the system, how do we maximize the common good?
It's simple, if you compare the utility of goods based on what you already have. Look at the S-shaped utility function that I just described for food and housing. A similar shape would probably obtain for education, as well, but let's base this one on units of money. Units of money are on the horizontal axis (0-6). The utility or usefulness of money is on the vertical axis (0-10).
Notice that if you have 1 unit of money, you might as well be dead, because the increase in utility to you is almost zero. You have 1 unit of money, and the bread costs 2 units. The utility of the money rapidly increases as it improves you chances for living. When you go from 1 to 2 units of money, utility increase by about 1 unit. Look how steep the growth in the usefulness of money becomes as you go from 2 to 3 units of money. With one extra unit of money, utility increases by 4 units! After securing life at 3 units of money, it continues to grow, but at a slowing rate. You gain 4 more units of utility going from 3 to 4 units of money, but after that, going from 4 to 5 units, you gain a paltry 1 unit of worth, and from 5 to 6, almost no extra value. When you are as rich as 6 units of money, more money can't improve life in any significant way.
In this view, increasing the common good boils down to this: Having people below 3 is not acceptable, ethically or pragmatically. It's wrong and creates conditions fostering crime or other risk seeking behaviors due to stressfulness. Investing tax dollars into getting people at least between levels 3 and 4 is what you want to shoot for minimally, because that is the point where return on investment is still great, but slightly diminishing. Also, the higher you go above 3, the more risk averse (conservative) the phenotype becomes. Times are good, and getting better. Why change? It is also where higher, or secondary levels of need, e.g., cognitive and spiritual, can be most usefully engaged.
What this usefulness function also tells us is that don't first give tax cuts for the rich. It doesn't add much to their utility. For the super rich, it adds nothing.