|Deep contemplation in general consists of thinking about a particular topic until it becomes clear to one and conceptually integrated into one’s soul.
Contemplation of God’s unity is a component of the daily prayer service.
It is what one must bear in mind when proclaiming, “Hear, Israel: The Lord is our God, the Lord is one” (Deut. 6:4).
The idea expressed in this verse is more than the fact that God is one and not two.
A person must be unwavering in his concentration, steadily focused for a period of time, envisioning and clarifying for himself the nature of this oneness, of the fact that “there is nothing besides Him” (Deut. 4:35); that not only is there no other like Him in terms of greatness or strength, but that nothing exists apart from Him.
This is not the kind of thought that a person can have fleetingly – “God exists; He is one” – and then go on with his day.
A person must meditate on this thought at length, reviewing it again and again.
Rabbi Adin Steinsaltz