Rabbi Nachman teaches,
Wisdom is the root of all things: “You made them all in wisdom” (Psalms 104:24) . Every person must therefore guard his mind against alien ideas. Fulfillment in life can come only through the pursuit of true wisdom – the wisdom of Godliness, the Torah. Compared to this, all other systems of wisdom are sheer emptiness; they are not wisdom at all.
When a person is born, his understanding is in a state of contraction. It only begins to develop when he starts using it to think about how to serve God. But when a person admits alien thoughts and ideas into his mind, the holiness of his mind and soul is diminished in direct proportion to the space they occupy. All kinds of negative character traits develop from this alien mentality and cluster around it.
This is why one must be so careful to protect one’s mind and not admit alien thoughts and ideas. To achieve true repentance and make amends for all your sins, make a determined effort to expel all alien thoughts from your mind. The conscious mind is a manifestation of the soul, and thus when a person sanctifies his mind and thereby his soul, everything is elevated and returns to its source. This is the essence of repentance.
It is not sufficient merely to guard yourself against alien ideas. Constantly try to bring new vitality into your mind as this will revive and refresh your soul. For the mind is a manifestation of the soul.
Likutey Moharan I, 35
With permission taken over from: Daily Dose Of Rabbi Nachman