Hadhrat Mufti-e-Azam Muhammad Shafi' (nawarrallahu marqaduhu) says,

"But the big difference of spiritual diseases and their cure is, that physical diseases can be realized by the eyes or other senses, they can be found out through the motion of pulse and tests of blood and faeces and the therapy can be done by medical instruments and medicine, too. But the spiritual diseases are not visible for the eyes, nor can they be recognized by the pulse or anything else, in the same way they are not cured by given aliments or medicines. The diagnosis and cure of these diseases can be only determined by the rules told by the Qur'an and Sunnah. The complete system for the Islah [rectification] of all the outward 'Amaal and Mu'amalat and the inner 'Aqaid and Akhlaq does exist in the Qur'an and Sunnah.

In the Ummah from the Sahabah and Tabi'een onwards till the Saliheen and Kamileen of the present age whosoever has gained any perfection then only through this system, the complete adherence to actions. The same way they would be adherent to Salah, Sauwm, Hajj and Zakat the exactly same way they would show perfection in Sidq [truthfulness], Ikhlas [sincerity], Tauheed [oneness of Allah], Tawwadhu [humility], Sabr [patience], Shukr [thankfulness], Tawakkul [thankfulness], Zuhd [asceticism] and other inner actions. The way they saved themselves out of fear from lies, fraud, stealing, shameless and other sins the exactly same way they would know Kibr [arrogance] and pride, disgracing and disreputing others, Hubb-e-Jah [love for fame], Hubb-e-Maal [love for wealth], Hirs [greed], Bukhl [being stingy] and other inner sins as forbidden and would stay abstinent from them."

[translated from "Dil ki Dunya"]