Meditation is always graded according to the aspirant and has to undergo various stages. One starts from thinking the body is Brahman, then the mind is Brahman till he realizes that none of these are actually Brahman.
Bhrigu is instructed by his father Varuna to realise Brahman.
Gradually Bhrigu pierced through these various layers of his
personality. From the five sheaths, he realizes that Bliss is Brahman,
and the physical, the vital, the mental and the intellectual sheaths
are not Brahman. Prāṇo brahmeti vyājānat (Tai.Upan. 3.3.1).
Talks with Ramana Maharshi, 146
When told that you are not the ego, realise the Reality. Why do you
still identify yourself with the ego? It is like saying, “Don’t think
of the monkey while taking medicine” - it is impossible. Similarly it
happens with common folk. When the Reality is mentioned why do you
continue to meditate Sivoham or Aham Brahmasmi? The significance must
be traced and understood. It is not enough to repeat the bare words or
think of them.
The meditation process is a gradual method of freeing consciousness from its entanglement in objects. Various Upanishads talks about various kinds of meditations: meditation on Vaishvanara, Hiranyagarbha, Ishvara, Omkara etc. but the ultimate purpose is Atma sakshatkara, or the realisation of Brahman.