When one meditates on a thought for a sustained period of time, beyond the moment in which it arose in one's mind, it becomes tangible and real.
That passing idea then becomes a meaningful thought that can build a complete structure.
Concentration and memory are like a song repeating its refrain within the words of prayer.
If a person has a fleeting thought of love or fear of God, and he takes that thought and sings it, repeats it again and again until he makes it a rhapsody, a tapestry, then he has turned an initial spark into a fervent flame.
–Rabbi Adin Steinsaltz
From The Thirteen Petalled Rose by Rabbi Adin Steinsaltz