Heraclitus once reckoned that you could not find the boundaries of the soul even if you followed every path, so deep was its measure. Today, it is scientifically unfashionable to speak of the soul. As Francis Crick so memorably put it, "You're nothing but a pack of neurons!"
Mental matters, however, are not so simple that, otherwise our brightest brains might have already hit on a third person theory that accounts for our first person subjectivity (if that isn't something of a contradiction). I prefer paradox . . . like Emily Dickenson's wonderful lines:
The brain is wider than the sky,
For, put them side by side,
The one the other will include
With ease, and you beside.
The brain is deeper than the sea,
For, hold them, blue to blue,
The one the other will absorb,
As sponges, buckets do.
The brain is just the weight of God,
For, lift them, pound for pound,
And they will differ, if they do,
As syllable from sound.
As Wittgenstein observed, "It is the I. It is the I, that is deeply mysterious."