Tawhid is a revolutionary concept and constitutes the essence of the teachings of Islam. It means that there is only One Supreme Lord of the universe. He is Omnipotent, Omniscient, Omnipresent and the Sustainer of the world. The Qur’an says: `Say: You may be sure that Allah is One. He is Needless (Independent). He begot none nor was He begotten. There is none like Him “. (Surah al‑Tawhid, 111:1 ‑ 4)
The prominent thinkers of the Muslim world hold that none being like Allah means His intrinsic Unity as propounded by the philosophers and the mystics.
The simplest way in which it can be described is this: When we say that Allah is without a like, it means that on principle there can be no partner of Him. It cannot even be supposed that there can be more than one God. Oneness is His absolutely essential and indispensable attribute.
Hence to be able to grasp the idea of His Unity, it is enough to have in mind the correct conception of Him. If we are conversant with the true meaning of this word, we shall automatically come to the conclusion that Allah is One. He cannot be more than one because plurality is inconsistent with His Existence.
Suppose there is a line which extends infinitely on both the sides. Suppose there is another line at a distance of a metre which runs parallel to the first and also extends infinitely in both the directions. There is no problem in supposing the existence of two such lines. That is why it is said that two parallel lines are those which are equidistant from each other throughout their length and never meet even if extended infinitely.
Irrespective of the controversy whether this definition of the parallel lines is correct or incorrect and absolute or relative, it is clear that it is possible to suppose the existence of two such lines.
Now let us suppose that there is a body which grows infinitely bigger and bigger in all its dimensions, its length, breadth and height. Now can we suppose the existence of another body beside the first, which also grows infinitely? No, we cannot, because the first body will fill all the space and no room will be left for the second one, whether finite or infinite, unless the second body penetrates into the first one. But is it possible for a body to penetrate into another body itself and not in the space between its molecules? Of course, not. Hence, it is not possible to suppose the simultaneous existence of two bodies infinite in all directions and in all their dimensions.
So far we have talked of infinite bodies. The supposition of the existence of one such body automatically negates the existence of another. But it does not negate the existence of something non‑corporeal. For example, it does not negate the existence of an infinite soul which should have penetrated into an infinite body.
Now let us consider a being infinite in every respect of existence. Is it possible to suppose the existence of two or more such beings? No, it is not, because if it is presumed that two such beings exist, the existence of each of them will be limited at least by that of another. As such neither of them will be infinite.
Hence Allah has no like or equal. On principle there cannot be two or more gods.
So far we have been able to know that there is a Creator, who is the Source of Existence and who is without a like. But is this the final limit of human knowledge about Him? Can we not go a step further and have some more knowledge of this Source of Existence?
Some scholars tend to believe that man can have only one `cognizance’, that is he can know that there is a source of existence, but further cognizance or knowledge is not within his reach.
These scholars hold that any name or attribute which may be used to express the Source of Existence with a view to add to the knowledge about Him, is likely to be totally unconnected with Him and will add only to one’s ignorance and misunderstanding.
According to this view the highest stage of knowledge which man can attain about the Creator is only that He exists and that He is above all that man can conceive or imagine. The cognizance of the Source of Existence proceeds only in one direction, viz. considering the source to be above all that human mind can conceive.
Evaluation of extremist view
The view to which these scholars tend is very attractive and of much value so far as it negates ‑all unreasonable and mythical ideas about Allah. But if we evaluate it from the realistic point of view, we find it somewhat extremist. If human knowledge about Allah is so limited that no reference can be made to Him except by means of the word `He’, which is absolutely vague, then how have we learnt that He really is?
It appears that the’ great scholars who have tended to this view, have mistaken complete and all‑round cognition for the relative one. A thing may have tens of characteristics which distinguish it from other things. By knowing anyone of these characteristics we can identify it and need not have a full knowledge of all its distinctive features. That is not the case with Allah alone, but the same principle applies to all that exists. For example, you have two children. You can recognize each of them easily.
But are you aware of all their physical and moral characteristics.
Hence if it is a question of an all‑round cognition of Allah, we must admit that it is humanly impossible.
But if it is a question of the cognizance of certain of His attributes and such relative knowledge as may distinguish Him from all others, of course man should have such knowledge so that he may be aware of His existence. As a matter of fact without having such knowledge, it is futile to talk of Allah.
Hence our inability to have a complete and all‑round cognition of a reality does not mean that we should express inability to have any. sort of knowledge of it. There is a middle stage, rather there are several intervening stages between an absolute and all‑round cognizance and an absolute and all‑round non‑cognizance viz. “relative cognition from one or more directions”.
A careful study of knowledge, its value and its limits shows that human information about this world is mostly relative. For this reason modern science is basically concerned with knowing the features of things and not their essence. The cognition of the Source of Existence also has similar limitations. When an intelligent and well informed person thinks of Allah, he exclaims from the core of his heart; “I don’t know what You are; You are what You are”.
However the same person when he looks at His signs and at a part of His distinctive marks, he gets somewhat acquainted with Him. Though this knowledge is far lesser than absolute cognition but, while possessing it, one can talk about `Him’ with certainty. 
It may be said that anybody who believes in Allah, identifies Him at least with one of His attributes by means of which he recognizes Him. The cognizance of Allah is accompanied at least by some such attributes as the Creator, the Sustainer,the Origin, the Self‑Existent etc.