How can we know something that we don't know anything about?
Begin with an appreciation of the Unconscious, a term first coined by Freud. Ultimately there is still much we do not understand beyond our conscious experiences, despite large research in neurobiology and neuropsychology.
It may sound contradictory, but consider learning to be more aware of our unconscious mental processes and how we are impacted by our own unconscious mental life. A popular Chinese saying goes like the above, (loosely translated) "to remove the grass, you have to get at the roots". This may well be universally understood as healing from within, and truly addressing many psychological difficulties through a deep understanding of our psychology. By learning more about how we are like in relation to our unconscious, we also learn about our defence mechanisms or manic defences, as Klein/Segal called it.
This video by the American Psychoanalytic Association aims to provide an interesting videographic to assist many potential clients or (even) clinicians alike to begin the journey of understanding the use of drilling down to what stirs and riles up aspects of ourselves that remain hidden. Enjoy.