Ultimate Good

Most systems of philosophy start by defining what good and evil mean.

In the interest of posterity, we'll do so here. This will allow us a common foundation to return when morality, as it inevitably does, manages to make itself a little vague and sticky.

Working from the bottom up is a known mathematical shortcut, and will be mentioned later. For now, let's see it in action in terms of good and evil.

What is, ultimately, good?
Happiness. Because all beings innately desire a fulfilling of their needs, however twisted the format may become, and feel joy when their needs are fulfilled.

What is Human good?
Human happiness. Because all humans are born with an equally blank moral slate, and deserve as much happiness in that moment as the next.

What is collective, Human good?
Universal happiness. Because most humans have empathy, and feel others' emotions as their own. Since everyone desires happiness, thus everyone should create happiness around them.

What is personal good?
Group happiness. Because we are affected by the group we surround ourselves with, and are made happier by their happiness. And in reverse, because our happiness affects others we have a moral duty to pursue personal happiness.

Thus, we can imagine a perfectly good action:
Any action that makes oneself happy, everyone around oneself happy, every Human happy, and every being happy.
This is not as easy as it may sound. In fact, it's pretty hard to do just one. But we have to do better.
Any action that makes every being happier in the long run.
Because brief happiness can overwhelm long-term suffering. But Humans cannot know everything, so:
Any action that is assumed to have a more positive influence on all beings in the long run.
Because we must play averages. And Humans can't do better than reason the effects of their actions, so:
Any action that is assumed to cause more happiness than harm for all beings in the long run, as reasoned given the information present at the time of the action.
And even then, one can't always be sure of the certainty of one's success.
Any action that is begun under the assumption it will cause more happiness than harm for all beings in the long run, as reasoned given the information present at the time of the action.
No loopholes.
Any action that is begun under the reasonable assumption it will cause more happiness than harm for all beings, including the possibility of its outcome from failure to success, in the long run as reasoned given the information present at the time of the action.

And so, a final, less rigorous statement of "good:"
An action that is believed to foster mostly happiness.