Those who are to reach the paths (magga) and fruitions (phala) leading to nibbāna will do so by way of the heart. If the heart isn’t trained, then no matter how much external goodness you may have, you won’t be able to reach nibbāna. Nibbāna can be attained only by training the heart in the practice of virtue, concentration, and discernment.
Virtue forms the basis for concentration; concentration, the basis for discernment; and discernment, the basis for release. Concentration is especially important because it forms the basis for discernment and intuitive understanding, which are the crucial factors of the path. You can’t do without concentration. If concentration is lacking, you can gain nothing but jumbled thoughts and obsessions, without any sound support.
Concentration is like a nail: and discernment, a hammer. If the nail isn’t held firm and straight, the hammer will strike it only hit-or-miss, and the nail will never get through the board. For the heart to penetrate the world and get through to the highest Dhamma, it must take a firm stance in concentration so as to give rise to intuitive understanding. Intuitive understanding can occur only to those who have centered the mind in concentration. As for discernment, it’s something we all have, but if it lacks intuitive understanding, it can never get beyond the world.
For this reason, we should all take an interest in the factors that form the path leading beyond suffering and stress to abundant wellbeing.
This reflection by Ajahn Lee Dhammadaro is from the book, Frames of Reference, (pdf) p.6, translated into English by Ṭhānissaro Bhikkhu.