How do we evaluate spiritual practices? (By "spiritual practices" here, I want to start by casting a big net--let's include everything from grace before meals to religious life.)
Easy answer--a spiritual practice in Christian tradition is good for us to the extent that it fosters our growth in ability to respond to Jesus' twofold commandment of loving God and neighbor.
Yeah, but--how do we KNOW we're becoming more loving?
This question is made tricky because the same spiritual practice that makes one person a saint can make another person worse. (E.g., volunteering in prison can make someone more compassionate or more judgmental, istm.)
Well, thank heaven for social scientists! David De Steno in the New York Times provided data for the moral efficacy of meditation. Astutely, he says that benefits that redound solely or mostly to the benefit of the meditator, (better productivity at work, or there are other studies that like meditation to lower blood pressure, less stress, et al.,) "weren’t of the utmost concern to Buddha and other early meditation teachers. As Buddha himself said, 'I teach one thing and one only: that is, suffering and the end of suffering.'” So he set out to test compassion. Test subjects were evaluated on their willingness to yield a seat to a visibly lame person--a very commonplace exercise of quotidian compassion. Drum roll, please:
The results were striking. Although only 16 percent of the nonmeditators gave up their seats — an admittedly disheartening fact — the proportion rose to 50 percent among those who had meditated.
And this was after only 2 months of practice.
In virtue ethics, we say that "practice makes perfect." If you want to be just, do just things, if you want to be courageous, practice courage. But it turns out that if you want to be compassionate, one route to that end is meditation.
So--clearly virtue ethics needs to be more alert to these "indirect" means of growing in virtue. There's also an implicit challenge to all our spiritual practices--do they make me better in visible ways, as meditation can make us compassionate?