Yoel 2:13

וְקִרְעוּ לְבַבְכֶם וְאַל-בִּגְדֵיכֶם, וְשׁוּבוּ אֶל-ה אֱלֹהֵיכֶם:  כִּי-חַנּוּן וְרַחוּם, הוּא--אֶרֶךְ אַפַּיִם וְרַב-חֶסֶד, וְנִחָם עַל-הָרָעָה

“And tear your heart, and not your garments, and return to the Lord your God; for He is gracious and compassionate, patient, and abundant in mercy, and regrets evil”

This passuk seems to be criticizing external actions and demanding people to be more truthful and sincere. However, we are frequently told that external acts are not worthless and will lead us eventually to the right motives. For example, in Pesachim 50 it says “A man should always occupy himself with Torah and the commandments, even if not for their own sake, for even if he does them with an ulterior motive, he will eventually come to do them for their own sake”.

Rav Dessler in Michtav M’Eliyahu (Chelek beis, in the section of Chodesh Av) discusses how fighting the yetzer hora on an external level can often be more successful than an internal attack . He describes how one employ tactics like telling the yetzer hora that he will sin, only later. Similarly, the Sefer Hachinuch in mitzvah 16 describes how even the most uncompassionate and cruel person will slowly change his nature if he forces himself to do acts of kindness.

Rav Dessler goes on to explain that although these methods are highly effective, some actions must be genuine. On Tisha B’av there is no value in acting sad and going through the motions if one has not contemplated why he should be sad. One most genuinely feel emotions, otherwise one’s actions are devoid of meaning and possibly even detrimental.

The Mesilas Yesharim (perek 22) explains the same applies to humility. If one acts in a humble manner before he appreciates how to acquire true humility, his actions are only to his detriment and make him a worse person. When one wants to be humble, it must start in his head with introspection and thought.

Yoel is telling us the same is true for teshuva. As Rav Dessler writes, there are no external or fake ways to do teshuva. One must spend time thinking about the past, eventually show remorse for one’s previous crimes, and beseech Hashem sincerely. This alone is the route one must take to achieve a complete teshuva. One must tear his heart and not his clothes.