अयमाकाशः सर्वेषां भूतानां मधु ।अस्याकाशस्य सर्वाणि भूतानि मधु ।यश्चायमस्मिन्नाकाशे तेजोमयोऽमृतमयः पुरुषो यश्चायमध्यात्मं हृद्याकाशस्तेजोमयोऽमृतमयः पुरुषोऽयमेव स योऽयमात्मा ।इदममृतमिदं ब्रह्मेदं सर्वम् ॥२,५.१० ॥
In plain words
This space is the honey of all beings, and all beings are the honey of this space. And the shining, immortal person who is in this space, and the shining, immortal person here in the space within the heart — he is this Self. This is the immortal, this is Brahman, this is all.
The commentators
Bṛhadāraṇyakopaniṣadbhāṣyam· bhāṣya
तथा आकाशः, अध्यात्मं हृद्याकाशः ॥
In plain words
It is the same with space: within you, its counterpart is the space inside your own heart.
Close to the Sanskrit
So too space; with reference to the self, the space in the heart.