From Emily Austin, associate professor in the Wake Forest Department of Philosophy:
Ancient Greek ethical philosophy starts from a psychological assumption that guides pretty much everyone: we want to be happy. How, you might think, can they equate ethics with happiness, since we seem to know so many happy people who are unethical in their daily lives? Likewise, we might know many morally upstanding people who are nevertheless wracked by unhappiness. The short answer is that the ancients didn’t mean the same thing by happiness (or eudaimonia) that we generally do.
First, the ancients thought happiness was objective, which means reporting one’s happiness is more like saying ‘Water is H2O’ than ‘I like pizza.’ There’s a fact of the matter about happiness in the same way that there’s a fact of the matter about water. This has the interesting result that you can be wrong about whether you are happy. If you say ‘I think water is H30,’ then someone can fairly respond that you are wrong. In the same way, the ancients think that someone can say, ‘I am happy,’ and find themselves subject to response, ‘No, you aren’t.’ For us, that’s a bit strange.
Second, all the ancient ethicists thought virtue is a necessary component of happiness. Without virtue, the most wealthy, beautiful, athletic and powerful person simply cannot be happy. To extend the comparison, water is not water with only one hydrogen atom.
Some of the ancients thought virtue required philosophical wisdom, while others thought it required only a sort of ethical prudence, but they all thought virtue was necessary. Picture your favorite tyrant, and the ancients will say that person is not virtuous regardless of what they say about themselves.
Finally, one big difference between the ancients and at least one popular conception of happiness is that the ancients were deeply suspicious of material goods, in part because material goods are likely to erode virtue and direct your attention to competition rather than cooperation. The more money you covet, the less attention you devote to virtue, all the while making yourself anxious about getting more and keeping what you have.
Some ethicists like Plato and the Stoics thought happiness required absolutely no material goods — virtue was sufficient for happiness. Others, like Aristotle, thought some material goods were necessary, but very few. Aristotle clearly thought you needed friends, but not much else.
So in sum, if you want to adopt an ancient attitude towards happiness, you need to think a person can be wrong about whether they are happy, that often their mistake lies in thinking an unvirtuous person can be happy, and that happy people don’t care much or at all about material goods. That, the ancients think, is the starting point of the happiness everyone really wants.