Friends:

In how many aspects are the five abilities (Indriya) purified?

There are these 5 mental Abilities:
The ability (faculty) of faith,
The ability of energy,
The ability of awareness,
The ability of concentration,
The ability of understanding.

These five abilities are purified in fifteen aspects.

When the noble friend avoids faithless persons, cultivates, frequents and honours
faithful persons, and reviews Suttas that inspire confidence, the faith ability is
purified in these three aspects.

When the noble friend avoids idle or lazy persons, cultivates, frequents, and honours
energetic persons, and reviews own practise of the 4 best Efforts, the energy ability
is purified in these three aspects.

When the noble friend avoids negligent, forgetful persons, cultivates, frequents and honours
mindful persons, and reviews the 4 Foundations of Awareness, the mindfulness ability is
purified in these three aspects.

When the noble friend avoids unconcentrated persons, cultivates, frequents and honours
focused & concentrated persons, and reviews the 4 Jhanas and Liberations, then the
concentration ability is purified in these three aspects.

When the noble friend avoids ignorant persons, cultivates, frequents, and honours persons
who understand, and reviews the insight of profound knowledge, the understanding ability
is purified in these three aspects.

So when the noble friend avoids these five kinds of persons, cultivates, frequents, & honours
these five kinds of persons, and reviews these five kinds of Suttas, the five abilities is purified
in these fifteen aspects.

Source: Sariputta in: The Path of Discrimination: Patidasambhidamagga:
http://www.pariyatti.com/book.cgi?prod_id=133494