Charles Ellicott Commentary Psalms 51:6

"Behold, thou desirest truth in the inward parts; And in the hidden part thou wilt make me to know wisdom." — Psalms 51:6 (ASV)

Truth. —Or, faithfulness.

Inward parts. —The Hebrew word is found only once elsewhere (Job 38:36), where it is in parallelism with “heart.”

The sincerity and true self-discernment which God requires can only come from spiritual insight, or, as the last clause states it, divine instruction.