What kind of practice truly makes merit never go to waste?

Elderly people, due to brain atrophy, have weakened supreme faculties (rūpa-prasāda), and the functions of the six consciousnesses become affected. What are these effects? For example, when they meet a new person, they remember them at the time but forget afterward—this is because the mind consciousness's power of recollection has declined. Is the function of the manas affected? The manas's own function is not affected, but its function of cooperating with the six consciousnesses is influenced by the six consciousnesses, not by the brain or the supreme faculty. Because when an elderly person sees that person, the impression does not penetrate deeply into the manas but remains only at the level of the mind consciousness, causing them to forget afterward.

However, an elderly person may have a deep impression of their close relative A, with very deep memories, and may even be constantly concerned about A in their heart. Even so, sometimes when seeing A, they clearly know in their mind that it is A, yet when they open their mouth they call out B's name, while still talking about matters concerning A. This is because the mind consciousness has become somewhat muddled, but the manas has not. The older a person gets, the weaker the function of the supreme faculty becomes, and the more muddled the mind consciousness becomes. Although the manas is not muddled, since bodily, verbal, and mental actions are all created by the mind consciousness together with the five sense consciousnesses, various kinds of confusion and obstruction are inevitably bound to occur.

In our Buddhist practice, in order that all efforts are not made for nothing and that the merit of our practice is not lost, how should we cultivate? We should not remain at the level of the mind consciousness's understanding, deliberation, analysis, recitation, and memorization, but should strive to develop meditative concentration (samādhi) and deliberate within that concentration, immersing the learned Dharma into the mind of the manas, and then, within concentration, letting the manas deliberate (manana) so as to attain realization through direct perception. Only by making more use of the manas can true wisdom be developed, allowing wisdom to grow continuously, extending into future lives, being enjoyed life after life, increasing life after life, and ultimately reaching perfect buddhahood.