What is the meaning of "purification of the innermost being" in the context of the levels of purity mentioned by Al-Ghazali, which are: purification of the outward, purification of the limbs, and purification of the heart?

What Abu Hamid intends by purifying the secret is the inner core of the heart from everything other than Allah Almighty, so that no thought other than Allah crosses it. This is the purity of the Prophets and the truthful ones. Ibn al-Qayyim mentioned that guarding thoughts is the foundation for the rectification of states, words, and deeds, and repelling them is easier than repelling intentions and resolutions once they have taken hold. The way to guard them is as follows: 1. Firm knowledge that Allah is aware of your heart and knows your thoughts. 2. Your shyness before Him, Glory be to Him. 3. Your veneration of Allah, that He should see such thoughts in His house, which He created for His knowledge and love. 4. Your fear of Him, that you might fall from His sight because of those thoughts. 5. Your preference for Him, that nothing but His love should reside in your heart. 6. Your apprehension that those thoughts might proliferate and consume the faith and love of Allah within the heart. 7. To know that those thoughts are like the bait thrown to catch a bird. 8. To know that those evil thoughts do not coexist with thoughts of faith and the calls of love and repentance. 9. To know that those thoughts are a sea of imagination with no shore; if the heart delves into its depths, it will drown. 10. That those thoughts are the valley of fools and the hopes of the ignorant, bearing nothing but regret and shame.

This is beneficial under two conditions: First: that it does not lead to abandoning any obligation or Sunnah. Second: that merely guarding them is not the sole objective; rather, one should empty the heart of those thoughts and fill it with their opposites, such as thoughts of faith, love, repentance, reliance on Allah, and awe.