VM 4.47 translated by Ven. Nanamoli wrote:However, what is particularly recommended is balancing faith with understanding, and concentration with energy. For one strong in faith and weak in understanding has confidence uncritically and groundlessly. One strong in understanding and weak in faith errs on the side of cunning and is as hard to cure as one sick of a disease caused by medicine. With the balancing of the two a man has confidence only when there are grounds for it.
Then idleness overpowers one strong in concentration and weak in energy, since concentration favours idleness. Agitation overpowers one strong in energy and weak in concentration, since energy favours agitation. But concentration coupled with energy cannot lapse into idleness, and energy coupled with concentration cannot lapse into agitation. So these two should be balanced; for absorption comes with the balancing of the two.
And from 4.49:
Strong mindfulness, however, is needed in all instances; for mindfulness protects the mind from lapsing into agitation through faith, energy and understanding, which favor agitation, and from lapsing into idleness through concentration, which favors idleness. So it is as desirable in all instances as a seasoning of salt in all sauces, as a prime minister in all the king's business.