We believe that this Primal Cause [God] is One. [His is] not like the oneness of a pair, nor like the oneness of a species, nor like man, whose complex oneness may be divided into many units, nor like the oneness of a simple body, which is one in number but may be divided and separated without end. Rather, He is One with a Oneness that knows no parallel in any manner. This is the Second Principle, as affirmed by the verse (Deut. 6:4): "Hear O Israel, God is our Lord, God is One."This ikkar, about God's unity, isn't a statement of monotheism, but rather a definition of the Jewish (original!) version of it. It posits that there is a complete difference between God and all other things in that only God is fully united.
A human being is the sum of its parts, organs, bones, cells, atoms. There are many, many ways of dividing a person into parts. In fact, it is an amazing feature of being a human that we are not really aware of the many parts that make us up. R. Aharon Soleveitchik wrote a very dramatically titled article called 'A Glimpse At Eternity From A Hospital Dungeon' in Tradition Vol. 21:3, Fall 1984. But he had a right to the dramatic title; he was then suffering from a terrible stroke. R. Soleveitchik, being a great man, took the opportunity from the hospital bed to contemplate and pen very interesting observations he made about the nature of the soul. He wrote that "Every time I raise my left arm or my left leg I feel a biological sensation analagous to the sensation I would perceive prior to the stroke whenever I picked up a heavy child. It is as if I had been competely detatched from my left leg and my left arm....how is it that even now I perceive the biological sensation of being a carrier of an arm and leg only when I raise my left arm or leg but not when I elevate my right arm or leg....It seems to me the let side is not "me"--only the right side of my body is "me"."
This very moving account reminds us that when all is well we simply don't perceive lifting our leg as lifting, I don't know, 20 pounds (?). But that is only because we are fortunate to perceive unity in our body.
According to the Rambam the above does not apply in any way to God, who could never, even in theory, be disunited in the way that we all are and that R. Soleveitchik personally felt. But as God isn't physical (stay tuned for the 3rd ikkar), a closer analogy may be our intangible side, our intellect, emotions etc. People are complex. How many times do we encounter people who have contradictory aspects to their personality? How many times has it been said that the late Shlomo Carlebach was a "complex" person? Of course all these contradictions co-exist within a person. Yet according to the Rambam the God whose "anger will blaze against you" and the God of "kol demama dakka" is one and the same, but unlike humans, there is no contradiction, no complexity, no disunity. Unlike people who at times are mad and at times are happy, God is always the same. And that is what is difficult about this ikkar.We can use all the prism analogies we want and we can explain that the Bible uses anthropomorphism, but it is, frankly, no small matter of reconciling the God of rahamim and din without recourse to compromising God's simplicity. If one could take a three-leaf clover, as St. Patrick did, and point to Irish pagans and say that God is a compound unity (why not 30,000 instead of 3?) that would be easy. However, that is not Judaism's God.
So do I believe it? To me what is compelling about the idea of the absolute unity of God unlike any other unity is that I think it is what is befitting of God. Mono theos, there is one God. Wouldn't the one God then be more one than one itself is (1 / 2 = .5)? Admittedly it isn't easy to swallow the dogmatic assertion that God's "anger" and God's "love" are literally the same thing. But to posit otherwise is not befitting God.