Kimiau Assa'adah

The book Kimiau Assa'adah by Imam Al Ghazali can be used as a reference

The Kimiau Assa'adah book, among its contents, discusses the Nature of the Heart & Spirit

As for your question, what is the nature of the heart, the Sharia does not explain it at length except in one verse:

"And they ask you about the spirit, say: "The spirit is among the affairs of my Lord."

(Q.S. al-Isra [17]: 85).

Because the spirit is part of divine power, namely from 'alam al-amr (the power of God's command) Allah SWT says:

"Remember, it is only Allah who creates and rules."

(Q.S. al-A'raf [7]: 54).

Thus, on the one hand humans are part of 'alam al-khalq (creation nature) and on the other hand part of 'alam al-amr. Everything that can be measured in terms of length, width and mechanism is included in 'alam al-khalq[6], but the heart does not have specific lengths and dimensions. Therefore, he did not receive distribution. If it can be divided, then it is included in 'alam al-khalq.

For example, in terms of being stupid, he becomes stupid and in terms of being smart, he becomes smart. But anything that consists of being stupid and smart at the same time is impossible. In other words, it is part of 'alam al-amr, because in 'alam al-amr there are no specific measurements for length, width and size.

Some of them think that the soul is qadim (beginning), so they are wrong. Others think that the spirit is 'ard (nature), so they are wrong, because nature never stands alone, but follows others.

So, the spirit is the origin of Adam's children, and the heart is where they grow. So, how could he be nature! Some groups say that the soul is a temporal body, they are also wrong, because the physical body receives division.

And the spirit that we have been calling the heart is a medium for knowing Allah. Therefore, it is not a body, nor a nature, but an element of angelic essence.

Knowing about the soul is very difficult [7], because religion does not provide the slightest way. And religion does not require knowing, because the essence of religion is sincerity (mujahadah), while ma'rifah (knowing) is a sign of guidance, as He says:

"And those who strive to (seek Our pleasure), We will truly show them Our ways."

(Q.S. al-Ankabut [29]: 69).

And whoever doesn't mean it, he shouldn't discuss it or search for the essence of the spirit. The main basis of mujahadah is knowing the army of the heart, because if a person does not know the ins and outs of the military, he is not justified in fighting jihad.

[6] Imam Qohtoby said that the spirit is not included in the category of KUN objects, meaning that the spirit is life itself. Life and life are the nature of the all-living One. The soul in the body is not a creature like the body. (See; aTaaruf Limadzhab expert in Sufism; alKalabadzi, p. 68, Darul scientific pole, Bairut).

[7] Said Imam Junaidy alBagdadi The spirit is something that is limited by the knowledge of Allah and no one understands it from His creatures. And you are not allowed to compare it to anything.