[Among the three essentials – Ethical Conduct (Sila), Mental Discipline (Samādhi) and Wisdom (Paññā)] Mental Discipline includes three other factors of the Eightfold Path: Right Effort, Right Mindfulness (or Attentiveness) and Right Concentration. (Nos. 6, 7 and 8 in the list).

Right Effort is the energetic will (1) to prevent evil and unwholesome states of mind from arising, and (2) to get rid of such evil and unwholesome states that have already arisen within a man, and also (3) to produce, to cause to arise, good and wholesome states of mind not yet arisen, and (4) to develop and bring to perfection the good and wholesome states of mind already
present in a man.

Right Mindfulness (or Attentiveness) is to be diligently aware, mindful and attentive with regard to (1) the activities of the body (kāya), (2) sensations or feelings (vedanā), (3) the activities of the mind (citta) and (4) ideas, thoughts, conceptions and things (dhamma). …

The third and last factor of Mental Discipline is Right Concentration leading to the four stages of Dhyāna, generally called trance or recueillement. In the first stage of Dhyāna, passionate desires and certain unwholesome thoughts like sensuous lust, ill-will, languor, worry, restlessness, and skeptical doubt are discarded, and feelings of joy and happiness are maintained, along with certain mental activities. In the second stage, all intellectual activities are suppressed, tranquility and ‘one-pointedness’ of mind developed, and the feelings of joy and happiness are still retained. In the third stage, the feeling of joy, which is an active sensation, also disappears, while the disposition of happiness still remains in addition to mindful equanimity. In the fourth stage of Dhyāna, all sensations, even of happiness and unhappiness, of joy and sorrow, disappear, only pure equanimity and awareness remaining.

Thus the mind is trained and disciplined and developed through Right Effort, Right Mindfulness, and Right Concentration.