The understanding of the higher “I” emerged as an opposition to the lower “I,” which traditionally correlates to the human ego, born in the mind. The higher “I” was supposed to be located outside the mind and represented our highest unchangeable individual nature. The individual soul – as something eternal and which everyone possesses – was quite suitable for that role. On the one hand, its presence explained the strange and inexplicable sense that there is no death, which is familiar to everyone, and on the other hand, is completely consistent with ancient versions of the creation of the world. Therefore, the version of the individual soul, which, on the one hand completely corresponded to the personality of the person and bore his sins, and on the other hand, was immortal, that is, unchangeable – existed a very long time, and to this day is very widespread.
If you look at the question of the soul at the level of commonly-accepted notions, then we must admit that if the eternal soul predetermines all the special features of a person’s individuality, all of his bad and good traits (after all, it is eternal, and that means it is incapable of change; after all, what can change cannot be eternal), then that means what is now defined as the ego is also part of the soul. And then it cannot be in any way the same as God, except for its immortality. And if it is really unchangeable, then all the whims of the body do not mean anything for it, and then it does not have the trait of a specific person, with whom it is seemingly united, and therefore there is totally nothing to judge it for at the Last Judgement.
This contradiction was realized perfectly by many mystics of ancient times, and they sought a way out through the complexification of the description of the construction of a person, introducing, for example, the concept of Spirit, which was higher than the soul and bore within it this very unchangeability and identity with God. But the level of common knowledge always predetermines the inability of more clear formulations and in general a deep clarification of the question. There simply do not exist suitable terms; there are no relevant concepts, and that is why mystics must get by with what there is, and call the rest inexpressible. Or think of some sort of terms of their own, but to raise them higher than the level of common knowledge does not work particularly well anyway, because all explanations are needed not so much by mystics as by those who listen to them.
In our time, for the definition of the eternal and unchangeable part of the human being, almost everyone uses the term “consciousness.” Undoubtedly, it reflects the situation far better than both the soul and Selfhood. But even here, far from everything is fine for the contemporary Advaitists.