Ahh Theravadins; they’re so triggered by emotions.
Well, first thing is that the Pali text on SC has this rather embarrassing mistake in these lines. It should be:
Mātā yathā niyaṃputtam
It is caused by the way the MS editors created the line breaks. In the source VRI edition it is correct, they have niyaṁ puttamāyusā. Worse, the MS editors added punctuation, which does not belong here at all, and they did so inconsistently, using comma in kp9 and dash in snp1.8.
As a strict policy, we do not make any changes to the Pali readings and text of our edition. However, we do sometimes make adjustments to punctuation, word breaks, and the like, and I think this is justified here. Here is the change.
On to the question! Here is a literal translation. I join the lines together, as it is impossible to translate keeping track with the half-lines in the MS edition.
Mātā yathā niyaṃ puttam āyusā ekaputtam anurakkhe;
Just as a mother would preserve with [her] life [her] own child, [her] only child,
Evampi sabbabhūtesu mānasaṃ bhāvaye aparimāṇaṃ.
so too [one] should develop a limitless heart for all creatures.
Mettañca sabbalokasmi mānasaṃ bhāvaye aparimāṇaṃ;
And [one] should develop a limitless heart of love for the whole world,
Uddhaṃ adho ca tiriyañca asambādhaṃ averamasapattaṃ.
above, below, and all around, unconstricted, free of hatred and enmity.
Just a few points of detail first.
Mānasa is an unusual poetic term equivalent to citta or ceto. Here it clearly plays the same role normally played by ceto in phrases like appamāṇā cetovimutti.
- The e ending in anurakkhe and bhāvaye is a poetic variant of the third person singular optative, more commonly encountered as eyya. The sense of this ending can be either “would” or “should”, and in translation I have varied it to suit the sense.
Anurakkha = anu (along) + rakkha (protect), but it is almost always used, not in the sense of “guard” like protecting against an enemy, but “preserve” in the sense of keeping something alive and present. In this sense, it is one of the four right efforts, the effort to “preserve” good qualities.
Now as to interpretation. This is, of course, a sutta on metta, and it is all about developing metta, both through one’s actions and through meditation. At this point in the sutta, we are moving from the part on the practice of meditation to the section on the metta heart’s release, i.e. jhana based on metta. And that is explicitly what the language of “limitless” refers to.
In literary analysis, we must distinguish between the direct statement and the metaphor. In the suttas, metaphors are not used to expand the meanings of direct statements, but to illustrate them. The overriding aim of the suttas is clarity of meaning. Normally the direct statements are in themselves perfectly clear, and the metaphor merely serves to reinforce and illuminate. This is sometimes obscured a little in verse, where the requirements of meter and the creativity of the poetic impulse pushes metaphors in less obvious directions. However, even there, this principle usually applies, it is just stretched a little.
So in this case, the direct statements are quite clear, even if couched in slightly unusual poetic terms. The meditator is encouraged to develop (bhāveti) a limitless heart of love; i.e. to make their metta grow, to make it become more, until it transcends any limitations. The direct statements in the verse are fully explicit and clear on this point, and they do not say that one should “protect” one’s mind of loving kindness.
To illustrate this point, the metaphor of the mother protecting her child is introduced. The “only child” is grammatically in the same case (accusative) as the “limitless mind”, that much is true; but they are governed by different verbs. The mother “protects” her child, the meditator “develops” their mind. As always, it would be a mistake to read the metaphor as changing the meaning of a clear direct statement.
It should go without saying—this being poetry and all—that the mother protects her child because of her love. Obviously the metaphor is pointing to a connection between the mother’s love and the love developed by the meditator. Equally obviously, a metaphor is only a metaphor, and a mother’s love is still limited to her child. But it is still the best example of worldly love that there is.
In the linked passage, Ven Thanissaro is quoted as saying:
It’s sometimes understood as saying that we should be willing to sacrifice our lives to protect all others, in the same way that a mother would sacrifice her life for the sake of her child.
For which a citation is required. As far as I can see, it is a straw man, I have never heard any Pali scholar say this.