DEUT140 ... four levels of acknowledging God's unity are to be found among men, according to their different levels of understanding and insight into this concept. The first [level] is the declaration of God's unity by the tongue alone. This level is attained by the child or the simple person who has no understanding of the meaning of belief, nor is its truth fixed firmly in his heart. The second level is the acknowledgment of God's unity with the heart and tongue, based on what one has received from tradition, because he believes the person from whom he has received it. lHowever,] one does not grasp [at this level] the true meaning of the subject on the strength of one's own intellect and understanding; rather, one is like a blind man who is led by one who can see. It may happen that one receives [the tradition] from someone who, likewise, knows it only from tradition. That would resemble a string of blind men, each of whom has his hand on the shoulder of the one in front of him, until the file reaches a person endowed with sight, who is at their head and guides them. Should this guide of theirs fail them and neglect to watch over them carefully, or if one of them should stumble or suffer an accident, then all of them would be affected: they would all stray from the path and either fall into a pit or a ditch, or blunder into an obstacle that would prevent them from continuing. The same is true of one who acknowledges God’s unity because he relies on tradition. One cannot be sure that he will not embrace polytheism, for when he hears the statements and arguments of the dualists, his views might change and he might fall into error without his being aware of it. For this reason our Masters have stated: “Be diligent in the study of Torah, and know what answer to give a heretic” (Avos 2:14). The third level is the acknowledgement of God's unity with the heart and tongue after one is able to support it [the doctrine] with arguments for the truth of His existence, by way of reasoned analysis. [However,] one does not have knowledge of the meaning of “the absolute one” and “the relative one.” A person [at this level] resembles a man, endowed with sight, who, on a journey to a distant land, reaches a point in the road where it branches off into many different directions. Although he knows the general direction and location of his city of destination, he does not know which road will take him there; despite great effort on his part, he fails to reach his destination, for he does not know the way. This is like what Scripture says: “A fool wearies himself with his efforts, for he does not know how to get to the city” (Koheles 10:15). The fourth level is the acknowledgement of God's unity with the heart and tongue after one knows how to support [the doctrine] with arguments and arrive at knowledge of His true oneness by way of analysis and correct, reasonable theories. This is the highest and most accomplished level of them all, and it is this high station which the prophet urged us to attain, when he said: “Understand it today and reflect on it in your heart: Hashem is the God in the heavens above and on the earth below, there is no other” (Devarim 4:39).