The differences between individual and individual and between generation and generation, between the souls of the patriarchs and the souls of our era, between great and small, are not differences in intelligence.
When we speak of the level of a great soul versus a minor soul, we are speaking of a particular quality-namely, the soul's receptiveness to holiness.
An individual who is neither wise nor learned can nevertheless possess an extremely lofty and holy soul.
On the other hand, a very wise and learned man might possess the lowliest of souls.
A person can be extremely accomplished in all areas and at the same time be completely dysfunctional in all that pertains to holiness.
A holy soul is a soul endowed with the very special sense and capacity of connecting to the divine.
And so in every generation there are the leaders of Israel, whose souls are in the category of "head" and "brain" in comparison with those of the masses and the ignorant.
Each individual generation is also a complete organism and includes souls of varying levels, from the souls of the leaders who constitute its head to the lowly souls whose spiritual stature is analogous to the heel's meager vitality in relation to the head.
The organism analogy relates not only to the difference in quality between the souls but also to their integration and interdependence.
All limbs of the generation are inexorably bound to the head in the same way that the limbs of a physical body are connected to, and vitalized by, the brain in the head.
–Rabbi Adin Steinsaltz
From Opening the Tanya, Chapter 2, by Rabbi Adin Steinsaltz