In plain words
This has been sung in many ways by the seers, separately in various Vedic texts, and decisively in reasoned statements that point to Brahman.
Word for word
[It has been] sung by the seers in many ways, separately by various Vedic texts, and also by the words that point to Brahman, reasoned [and] decisive.
ऋषिभिर्बहुधा गीतं छन्दोभिर्विविधैः पृथक्ब्रह्मसूत्रपदैश्चैव हेतुमद्भिर्विनिश्चितैः
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
तत् क्षेत्रक्षेत्रज्ञयाथात्म्यं विवक्षितं स्तौति श्रोतृबुद्धिप्ररोचनार्थम् —
ऋषिभिः वसिष्ठादिभिः बहुधा बहुप्रकारं गीतं कथितम् । छन्दोभिः छन्दांसि ऋगादीनि तैः छन्दोभिः विविधैः नानाभावैः नानाप्रकारैः पृथक् विवेकतः गीतम् । किञ्च, ब्रह्मसूत्रपदैश्च एव ब्रह्मणः सूचकानि वाक्यानि ब्रह्मसूत्राणि तैः पद्यते गम्यते ज्ञायते इति तानि पदानि उच्यन्ते तैरेव च क्षेत्रक्षेत्रज्ञयाथात्म्यम् ‘गीतम्’ इति अनुवर्तते । ‘आत्मेत्येवोपासीत’(बृ. उ. १ । ४ । ७) इत्येवमादिभिः ब्रह्मसूत्रपदैः आत्मा ज्ञायते, हेतुमद्भिः युक्तियुक्तैः विनिश्चितैः निःसंशयरूपैः निश्चितप्रत्ययोत्पादकैः इत्यर्थः ॥ ४ ॥
In plain words
What he means to lay out — the field and the knower of the field, exactly as they are — he first praises, to give the listener's mind an appetite for it.
By the sages — Vasiṣṭha and men like him — it has been sung again and again, in countless forms; 'sung' here simply means 'taught.' By the hymns of the Veda too — the Ṛg and the rest — sung in their many ways, their many moods, their many shapes, each hymn taking it up on its own, distinctly. And beyond that, by the very sentences that point straight at Brahman — what we call the Brahmasūtras. Why 'sūtra'? Because through them one arrives, one comes to know. That is why they are called 'padas' — footholds. And it is these same footholds that also sing the true nature of field and knower; the word 'sung' simply carries forward and applies here as well. Take a line like 'One should meditate on it precisely as the Self' (Bṛhadāraṇyaka Upaniṣad 1.4.7) — through words like that, the Self becomes known: words that carry their own reasons, that stand on real logic, that settle the matter for good, that leave no doubt standing, that plant a firm conviction and hold it there. That is the meaning.
Close to the Sanskrit
That true nature of the field and the knower of the field, which is intended to be stated, he praises — for the purpose of kindling the hearer's intellect's appetite for it.
By the sages — Vasiṣṭha and others — [it has been] sung in many ways, in many forms; 'sung' [means] 'narrated'. By the metres — [that is] the Vedic hymns, the Ṛgveda and so on — by those hymns, in various ways, with various modes, with various kinds, separately, [that is] distinctly, [it has been] sung. Moreover, by the words of the Brahmasūtras too, indeed — sentences indicative of Brahman are called Brahmasūtras; by them one 'goes', [that is] is led, is come to know — thus those are called 'padas'; by those very words too, the true nature of the field and the field-knower [is likewise] 'sung' — [the word] 'sung' continues [to apply] here. By such words beginning with 'One should meditate on it precisely as the Self' (Bṛhadāraṇyaka Upaniṣad 1.4.7), the Self is known — [words that are] possessing reasons, endowed with logical argument, firmly determined, of the nature free from doubt, productive of a settled conviction — such is the meaning.