Deep contemplation consists of thinking about a topic until it grows as clear as possible.
Contemplation of the divine unity is part of the daily prayer service, as we proclaim, "Hear O Israel, the Lord our God, the Lord is One," a thought that goes beyond stating that He is numerically one.
This idea must be held for a length of time, steadily and continuously, so that we visualize and understand the nature of that oneness, which is that there is no other than He.
Not only is there no one like Him, as great or as strong, but there is no existence but His.
This is not a realization that one can reach in an instant – God exists; God is One; very nice – and go on dealing with one's business.
It demands profound study and continuous, repeated consideration, as one repeatedly considers the forms and postulates, the questions and conclusions that pertain to this concept.
–Rabbi Adin Steinsaltz
From The Long Shorter Way by Rabbi Adin Steinsaltz