It’s not clear to me that the doctrine, taken in a robust sense, is even coherent. To see why, consider the following argument, which ultimately comes from the Shaykh (though in a different context). The version of it I make use of here is from II.5.243 of the Hikmat al-Ishrāq, where Suhrawardi writes:
.و لا تظنن ان الانوار المجردة تصير بعد المفارقة شيئا واحدا, فان شيئين لا يصير ان واحدا
“Do not opine that the immaterial lights become one thing after separation [i.e., from the body]; for two things do not become one.”
That is to say, for any two entities that become one or unified, exactly what is it for the one to become unified with the other or for the two to become one? For now, never mind that, ontologically, upholders of fanā’ presuppose that, fundamentally, there is only one reality and that reality is God or existence or whatever (expressed in many ways, under different concepts). Grant also their other claim i.e., that the way this ultimate union is achieved is through a dissolving of the ego (ana) – the prism, they say, that refracts, as it were, the one reality into a multiplicity – by means of sustained spiritual exercises, until eventually it’s totally annihilated (fana), at which point ittihād!
“The man who says Ana’l-‘abd, “I am the servant of God” affirms two existences, his own and God’s, but he that says Ana’l-Haqq,“I am God”, i.e., “I am naught, He is all: there is no being but God’s.” This is the extreme of humility and self-abasement.” (Rumi 1995: 184, tr. Nicholson)
Still, even assuming all that, just how is the consequent ‘oneness’ with the ultimate reality effected? Taking my cue from the Shaykh al-Ishrāq, when, say at the crucial moment, x is annihilated, and a ‘becoming one’ with y occurs, after the unification either:
a) both x and y subsist (ibid., 243.12-3)
b) neither x nor y subsists (243.13-4), or
c) y subsists but x doesn’t (or vice versa) (243.14-5)
Whatever alternative we opt for, and they seem to me to be exhaustive, there’s no unification or a ‘becoming one’ of two things. In a), whatever other way x (read: ego) may have been altered (spiritually or otherwise, whether it’s now the ‘higher self’ or not), the point is it’s something that retains a distinct huwiyya (identity) after the crucial moment. As such, there’s no unification. Option b) can be sub-divided; after both x and y cease, either (b.1) nothing remains or (b.2) something else z remains. If (b.1), we don’t have the unity of two things, just their ceasing to exist, period. If (b.2), why isn’t that just x and y ceasing to exist and a new thing z coming to exist as a result? Hence, no unity of two here either. Finally, with c), it’s self-evident why no unification occurred. Therefore, the whole doctrine interpreted as such just doesn’t make sense.
On the basis of this conclusion, I state a general rule (hukm): poetic and ecstatic claims (shatahāt) which suggest ittihād ought to be submitted to sober analyses in a way that will reveal their harmony with the above results.