Ghatakasha – Mahakasha are terms used in Vedanta philosophy to explain the identity of the soul and the over-arching soul – atman and Brahman. Ghatakasha is akasha (ether) in a jar, whereas mahakasha is the universal ether, which is represents the oneness of atman.
According to Vedanta philosophy, atman is one though we speak of many individual souls. The examples of ether in a jar and the universal ether are given to explain the oneness of atman and the unreal appearance of many individual souls.
The appearance of many individual souls is compared with the ether inside a jar. The ether in a jar is not different from the ether outside the jar. So also the atman (individual soul) is not different from the soul outside (Brahman). The difference of the ether is observed because it is specified in relation to the jar, but outside the jar, the universal ether is not specified. It is unlimited. In the same manner, the jivatmans (individual souls) appear different due to avidya (ignorance).
If the jar breaks, the ether mingles with the universal ether and becomes one, so also the individual souls become one with Brahman as soon as ignorance is removed. No longer is there any difference between the two.
Brahmasutra states – Akashatalingata (the word) akasha (ether) is Brahman on account of the characteristic marks of tat (Brahman). The pot (or even the eye of needle) makes the difference between ether within and without. With the dawn of vidya (knowledge), we know that there is no difference at all.