Sayyid Muhammad Hussain Tabataba'I
Every reality in this world which we can possibly imagine is a limited reality, that is, one whose actualization depends upon certain necessary causes and conditions. If these do not exist that reality cannot exist in the world. Every reality has a boundary beyond which it cannot extend its existence. Only God is such that He has no limit or boundary, for His reality is absolute and He exists in His Infinity no matter how we try to conceive of Him. His Being does not depend upon and is not in need of any causes or conditions. It is clear that in the case of something limitless we cannot conceive of multiplicity, for any supposedly second reality will be other than the first, as a result of which each would be limited and bound and would set a boundary to the reality of the other. For example, if we consider a limitless volume we cannot conceive another limitless volume alongside it. And if we do suppose another, it will be the same as the first. Therefore, God is one and has no partner.
We have already mentioned the Bedouin who approached Ali in the middle of the fighting during the Battle of the Camel and asked if he asserted that God was one. In answer Ali said, "To say that God is one has four meanings: Two of those meanings are false and two correct. As for the two incorrect meanings, one is that one should say 'God is one' and be thinking of number and counting. This meaning is false because that which has no second cannot enter into the category of number. Do you not see that those who said that God is the third of a trinity [i.e., the Christians] fell into infidelity? Another meaning is to say that so and so is one of this people, namely a species of this genus or a member of this species. This meaning is also not correct when applied to God, for it implies likening something to God and God is above all likeness.
"As for the two meanings which are correct when applied to God, one is that it should be said that God is one in the sense that there is no likeness unto Him among things. God possesses such uniqueness. And one is to say that God is one in the sense that there is no multiplicity or division conceivable in Him, neither outwardly nor in the mind nor in the imagination. God possesses such a unity." (Bihar al-anwar, vol. II, p. 65)
Ali has also said, "To know God is to know His Oneness." (Bihar al-anwar, vol. II, p. 186) This means that the Being of God is unlimited and infinite suffices to prove His Oneness, for to conceive a second for the Infinite is impossible. There is therefore no need of any other proofs, although there exist many others.