Building on conviction, you then put forth the persistent effort to develop within yourself whatever’s going to be skillful on the path and to abandon anything unskillful that will get in the way of the path.
Mindfulness is what remembers what’s skillful and what’s not skillful. It also remembers what to do with skillful qualities and unskillful qualities when they are present in the mind—and how to develop skillful qualities when they aren’t.
Concentration builds on persistence and mindfulness in that, when skillful qualities are fully developed, they lead the mind to a state of stillness and peace, together with a sense of deep inner well-being. The stillness then allows you to detect things in the mind that you can’t notice when it’s running around. The sense of well-being gives you the strength needed to nourish the mind in order to keep on the path.
Supported and nourished in this way, discernment then checks the results of what you’re doing to see how they can be improved. In this way, it then feeds back into the other qualities as well, strengthening your conviction, persistence, mindfulness, and concentration.
In the Buddha’s image, the first four qualities are like the rafters you put up to support a roof, while discernment is the ridgepole that connects them all and makes them firm. The ridgepole relies on the rafters, but it also ties them together so that they’re solid and tight.
This reflection by Ajaan Geoff is from the book, The Five Faculties, (pdf) pp.8-9.