"Lovingkindness (metta) is the way to purity
for one who has much ill will,
compassion (karuna) is the way to purity
for one who has much cruelty,
gladness (mudita) is the way to purity
for one who has much aversion (boredom),
and equanimity (upekkha) is the way to purity
for one who has much greed.
Also attention given to beings is only fourfold,
that is to say,
as bringing welfare,
as removing suffering,
as being glad at their success,
and as unconcern, [that is to say, impartial neutrality.]
And one abiding in the Measureless States
should practice lovingkindness and the rest
like a mother with four sons,
namely,
a child,
an invalid,
one in the flush of youth,
and one busy with his own affairs;
for she wants the child to grow up,
wants the invalid to get well,
wants the one in the flush of youth to enjoy for long the benefits of youth,
and is not at all bothered about the one who is busy with his own affairs.
That is why the Measureless States are only four
as `due to paths of purity and the other sets of four'.
From: Visuddhi Magga IX 108
The Path to Purification Chapter IX The Divine Abidings
By Bhadantacariya Buddhaghosa
Translated from the Pali by Bhikkhu Nanamoli

With metta / Antony.