The greatest deficiency in yir’at shamayim (fear of heaven) that is not well connected to the light of Torah is that fear of thought replaces fear of sin. Because a human being begins to be afraid of thinking, he drowns in the morass of ignorance, which robs him of the light of soul, weakens his vigor, and casts a pall over his spirit.

Orot ha-Qodesh, vol. 3, pg. 26,  

Rabbi Avraham Kook