Here is Bhagavan’s brief account of how he resolved this question during his ego-death experience in 1896 in Madurai.

When I scrutinised within the mind ‘Who is the seer?’ the seer became non-existent and I saw that which remained. The mind does not [now] rise to say ‘I saw’; how [therefore] can the mind rise to say ‘I did not see’. (Arunachala Ashtakam, verse two, tr. Sadhu Om)

Questioning ‘Who am I?’ within one’s mind, when one reaches the Heart the individual ‘I’ sinks crestfallen, and at once reality manifests itself as ‘I-I’. Though it reveals itself thus, it is not the ego ‘I’, but the perfect being, the Self Absolute. (Ulladu Narpadu, verse 30)

‘Whence does this ‘I’ arise? Seek this within. This ‘I’ then vanishes. This is the pursuit of wisdom. Where the ‘I’ vanished, there appears an ‘I-I’ by itself. This is the infinite [poornam]. (Upadesa Undiyar, verses 19 and 20).