Bhashya
किं च तेन हि चैतन्यात्मज्योतिषा अवभास्यमानः प्राणः आत्मभूतेन प्राणिति, तेन प्राणस्यापि प्राणः सः, तं प्राणस्य प्राणम्; तथा चक्षुषोऽपि चक्षुः; उत श्रोत्रस्यापि श्रोत्रम्; ब्रह्मशक्त्यधिष्ठितानां हि चक्षुरादीनां दर्शनादिसामर्थ्यम्; स्वतः काष्ठलोष्टसमानि हि तानि चैतन्यात्मज्योतिःशून्यानि; मनसोऽपि मनः— इति ये विदुः— चक्षुरादिव्यापारानुमि-तास्तित्वं प्रत्यगात्मानम्, न विषयभूतम् ये विदुः—ते निचि-क्युः निश्चयेन ज्ञातवन्तः ब्रह्म, पुराणं चिरंतनम्, अग्र्यम् अग्रे भवम् ।‘तद्यदात्मविदो विदुः’ इति ह्याथर्वणे ॥
In plain words
Here is why: lit by that light which is pure consciousness itself, the breath breathes as if it had become the self. That is exactly why it is called the breath behind the breath. In the same way it is the eye behind the eye, the ear behind the ear. The eye and the other senses can see and hear at all only because Brahman's power stands behind them; left to themselves they are as inert as a log or a clod of dirt, with no trace of that conscious light in them. And it is the mind behind the mind too. Those who know it this way — as the innermost self, whose presence you infer only from the working of the eye and the rest, never as something you could look at directly — they are the ones who have truly grasped Brahman: ancient, age-old, first of all things, existing before everything else. As the Atharva tradition puts it: that is what the knowers of the self know.
Close to the Sanskrit
Moreover, because the breath, illumined by that light whose nature is consciousness, breathes as though it had become the self by that light — for that reason that light is itself the breath of the breath; hence 'the breath of breath.' Likewise it is 'the eye of the eye,' and 'the ear of the ear.' For the capacity for seeing and so on belonging to the eye and the other organs exists because they are governed by the power of Brahman; by themselves they are, like a piece of wood or a clod of earth, devoid of the light whose nature is consciousness. And 'the mind of mind' — thus those who know the inward self, whose existence is inferred only through the functioning of the eye and the other organs, and not as an object of perception — those who know thus have truly ascertained with certainty Brahman, the ancient, the age-old, the foremost, that which exists prior to all. For it is said in the Atharvana: 'that which the knowers of the self know.'