KAṬHA UPANISHAD · 2.3.13

अस्तीत्येवोपलब्धव्यस्तत्त्वभावेन चोभयोः ।
अस्तीत्येवोपलब्धस्य तत्त्वभावः प्रसीदति ॥ १३॥

astītyevopalabdhavyastattvabhāvena cobhayoḥ ।
astītyevopalabdhasya tattvabhāvaḥ prasīdati ॥ 13॥

One must apprehend Him in the concept “He is” and also in His essential principle, but when he hath grasped Him as the Is, then the essential of Him dawneth upon a man.