I take the following extracts from the book titled, 'The Secrets of Secrets', by Shaykh Abdul Qadir Jilani (RA), which has been translated to english (and interpreted) by Shaykh Tosun Bayrak al-Jerrahi al-Halveti:
On Fasting Prescribed the Religion and On Spiritual Fasting
The fasting perscribed by the religion is to abstain from eating and drinking and sexual intercourse from dawn to sunset, while spiritual fasting is, in addition, to protect all the senses and thoughts from all that is unlawful. It is to abandon all that is disharmonious, inwardly as well as outwardly. The slightest breach of that intention breaks the fast. Religious fasting is limited by time, while spiritual fasting is forever and lasts throughout one's temporal and eternal life. This is true fasting.
Our Master the Prophet (SAW) says, 'There are many of those who fast who get only hunger and thirst for their efforts and no other benefit'. There are also those who break their fast when they eat, and those whose fast continues even after they have eaten. These are the ones who keep their senses and thoughts free of evil and their hands and their tongues from hurting others. It is for these that Allah Most High promises, 'Fasting is a deed done for My sake, and I am the one who gives its reward.' About the two kinds of fasting, our Master the Prophet (SAW) says, 'The one who fasts has two satisfactions. One is when he breaks his fast at the end of the day. The other is when he sees.'
those who know the outer form of the religion say that the first satisfaction of the one who fasts is the pleasure of eating after a day of fasting, and the meaning of the satisfaction 'when he sees' is when someone who fasted the whole month of Ramadan sees the new moon marking the end of the fast and beginning the festivities of the holiday. The ones who know the inner meaning of fasting say that the joy of breaking the fast is the day that the believer will enter Paradise and partake of the delights therein, and the meaning of the greater joy of seeing is when the faithful sees the truth of Allah with the secret eye of his heart.
Worthier than these two kinds of fasting is the fast of truth, which is preventing the heart from worshipping anything other than the Essence of Allah. It is performed by rendering the eye of the heart blind to all that exists, even in the secret realms outside of this world, except the love of Allah. For although Allah has created all and everything for man, he has created man only for Himself, adn He says 'Man is My secret and I am his secret'. That secret is a light from the devine light of Allah. It is the centre of the heart, made out of the finest matter. It is the soul which knows all the secret truths; it is the secret connection between the created one and his Creator. That secret does not love nor lean towards anything other than Allah.
There is nothing worthy to wish for, there is no other goal, no other beloved in this world and in the hereafter, except Allah. If an atom of anything other than the love of Allah enters the heart, the fast of truth, the true fast, is broken. Then one has to make it up, to revive that wish and intention, to return back to His love, here and in the hereafter. For Allah says: 'Fasting is only for Me, and only I give its reward'.