यो ह वै प्रतिष्ठां वेद प्रतितिष्ठति समे ।प्रतितिष्ठति दुर्गे ।चक्षुर्वै प्रतिष्ठा ।चक्षुषा हि समे च दुर्गे च प्रतितिष्ठति ।प्रतितिष्ठति समे प्रतितिष्ठति दुर्गे ।य एवं वेद ॥बृह. ६,१.३ ॥
In plain words
He who knows the foundation stands firm on level ground and stands firm on rough ground. The eye is the foundation, for by the eye one stands firm on level and on rough ground alike. He stands firm on level ground and on rough ground, who knows this.
The commentators
Bṛhadāraṇyakopaniṣadbhāṣyam· bhāṣya
यो ह वै प्रतिष्ठां वेद, प्रतितिष्ठत्यनयेति प्रतिष्ठा, तां प्रतिष्ठां प्रतिष्ठागुणवतीं यो वेद, तस्य एतत्फलम् ; प्रतिति- ष्ठति समे देशे काले च ; तथा दुर्गे विषमे च दुर्गमने च देशे दुर्भिक्षादौ वा काले विषमे । यद्येवमुच्यताम्, कासौ प्रतिष्ठा ; चक्षुर्वै प्रतिष्ठा ; कथं चक्षुषः प्रतिष्ठात्वमित्याह — चक्षुषा हि समे च दुर्गे च दृष्ट्वा प्रतितिष्ठति । अतोऽनुरूपं फलम्, प्रतितिष्ठति समे, प्रतितिष्ठति दुर्गे, य एवं वेदेति ॥
In plain words
Whoever truly knows what a foothold is — knows that it's called that because "one stands firm by means of it" — whoever knows that quality as his own gets exactly this: he stands steady on level ground and in easy circumstances, and just as steady on uneven ground, on terrain that's hard to cross, in a place hit by famine, or in any hard season. But then — what actually is this foothold? The eye is the foothold. How can the eye be that? Because it's by seeing with the eye that a person keeps their footing, on smooth ground and on rough ground alike. So the reward matches exactly: he stands firm on level ground, stands firm on rough ground, simply by knowing this.
Close to the Sanskrit
He who indeed knows the foothold [knows that] 'one stands firm by means of this' — thus [it is called] the foothold; that foothold, possessed of the quality of being a foothold, he who knows — his is this fruit: he stands firm on level ground and in [smooth] circumstance; likewise on uneven ground and in terrain hard to traverse, and in a place [afflicted] by famine and the like, or in a difficult circumstance. If it be so said, [one may ask]: what is this foothold? The eye, verily, is the foothold. How is it that the eye has the nature of being a foothold? To this he says: because, having seen by means of the eye, one stands firm both on level ground and in difficult terrain. Hence the fruit that matches: he stands firm on level ground, stands firm in difficult terrain — he who knows thus.