Bhashya
तथा यत्किंच अविज्ञातं विज्ञानागोचरं न च संदिह्यमा- नम्, प्राणस्य तद्रूपम् ; प्राणो ह्यविज्ञातः अविज्ञातरूपः हि यस्मात् प्राणः—अनिरुक्तश्रुतेः ।विज्ञातविजिज्ञास्याविज्ञातभे- देन वाङ्मनःप्राणविभागे स्थिते त्रयो लोका इत्यादयो वाच- निका एव ।सर्वत्र विज्ञातादिरूपदर्शनाद्वचनादेव नियमः स्मर्तव्यः ।प्राण एनं तद्भूत्वावति—अविज्ञातरूपेणैवास्य प्रा- णोऽन्नं भवतीत्यर्थः ।शिष्यपुत्रादिभिः संदिह्यमानाविज्ञातो- पकारा अप्याचार्यपित्रादयो दृश्यन्ते ; तथा मनःप्राणयोरपि संदिह्यमानाविज्ञातयोरन्नत्वोपपत्तिः ॥
In plain words
Similarly, whatever remains uncomprehended—lying beyond direct cognition and yet not an object of doubt—belongs to the form of prāṇa. For prāṇa is inherently unperceived, as confirmed by the Vedic passage concerning the undefined. Once speech, mind, and prāṇa are classified under the threefold categories of the known, the to-be-known, and the unknown, scriptural declarations like 'The three worlds' stand as authoritative testimony. This rule of meditation is to be understood directly from the sacred texts, since the respective forms of the known, the to-be-known, and the unknown are observed everywhere. The statement that 'Prāṇa protects this person by becoming that' signifies that prāṇa serves as food for him precisely in its unperceived character. Just as preceptors and fathers are sometimes not fully recognized in all their benevolent actions by their disciples or children, so too is it entirely appropriate for the mind and prāṇa to function as nourishing food even when their operations remain subtle and unknown.
Close to the Sanskrit
Similarly, whatever is unknown—beyond the reach of knowledge and not subject to doubt—is the form of prāṇa; for prāṇa is indeed unknown, having an unknown nature because of the scriptural text about the undefined. When the division of speech, mind, and prāṇa is established through the distinction of the known, the to-be-known, and the unknown, statements such as 'The three worlds' are purely scriptural. The rule is to be remembered solely from the scriptural text, due to seeing the forms of the known and so forth everywhere. 'Prāṇa protects this person, having become that'—meaning that prāṇa becomes food for him precisely in the form of the unknown. Teachers, fathers, and others whose helpfulness is doubted or unknown by disciples, sons, and others are also seen; similarly, the appropriateness of mind and prāṇa acting as food, though they are doubted and unknown, is fully justified.