What is the difference between these two expressions? Wouldn't it be more logical to say that God is Unique instead of One?1
In terms of the uniqueness of God, Yajid (Unique) is much deeper than Ejad (One), because Ejad It gives rise to more than one thing, whereas Yajid, It does not allow for more than one.
Even so, the concept of Ejad It represents that God is "One" even in this creation that appears multiple. Even where there seems to be multiplicity, however, God is One.
If only I would say Yajid (Unique), would represent that God is unique and there is not even a possibility for us to live, in quotes, "separate" from God, or to have the feeling that we live separate.
So, it is used Ejad This is meant to show us that, although there is a world and we see the world, nevertheless, there is only one God and the whole world is one with God.
It is obvious that God is above the world; in Hasidism, the difference between Makif y Pnimí.
Makif It means something transcendental and Pnimí an inner meaning. In the forces of the soul, for example, the will is Makif because it affects the whole body equally. Pnimí It is, for example, the intellect, vision or hearing, which have a specific place in the body where they function.
God is Makif y Pnimí At the same time. You can't say that God is only "outside" or "above" the world or only "inside" it; both things are real at the same time. There is a paradox, yes, because God is infinite and can do whatever He wants.
In the book Avodat HaKodesh2, From Meir ibn Gabbai, the idea emerges that just as God has power in the infinite, God also has power in the finite. For if we say that He is only infinite, then He is not complete, and if He is only finite, neither is He complete. Both at the same time are a paradox, but God, by definition, is a paradox.
If we limit God to being like a human being, then why would I want a God who is as "foolish" as me? I have myself for that, which is what atheists do: "I am my own god." But God transcends what we can understand. God is both infinite and finite.
Now, if we say that it is Yajid (Unique), is like saying that it transcends the world, that it is infinite but not in the finite. When we say Ejad (One), our sages explain (and this is mentioned in the Code of Jewish Law) the meaning of the letters:
The Aleph It represents the "One", the Master of the world.
The Jet (which is worth 8) represents the seven heavens and the earth.
The Dalet (which is worth 4) represents the four directions of the world.
What we are saying is that all directions, the heavens and the earth are one with God. Despite the multiplicity we see (people, animals, plants), God is united with this world and within it. He is not only transcendental (Makif), but it is internal (Pnimí) at the same time. That is the fundamental difference. That is why the true oneness of God is expressed with the word Ejad - One.
1Based on Hasidic discourse Kol haMaaríj beEjad 5700 (Sefer HaMaamarím 5700, p. 140).
2Part 1, Chapter 8.