The "midst" of something in the Bible represents the thing that is most central and most important to the spiritual state being described, the motivation that drives everything else. In general this will be something we love or feel, because at the core of things we are what we love; our loves define us.

(Odkazy: Apocalypse Revealed 44, 90, 268; Arcana Coelestia 2252, 2940, 10153, 10365, 10557, 10635, 10641)

507. And all green grass was burnt up, signifies that all true knowledge [scientificum] was destroyed by the cupidities of the same loves. This is evident from the signification of "grass," as being knowledge [scientificum] (of which presently); also from the signification of "green," as being truth and living from truth, because as green grass serves as food for animals, so true knowledge serves for spiritual nourishment for man; for whatever is produced in fields, in gardens, and in plains, and serves for nourishment either for man or beast, has a correspondence with such things as serve for the nourishment of the spirit and mind, and such nourishment is called spiritual nourishment. Like things appear in the spiritual world, from the correspondence of spiritual things with natural things; and as the Word in the letter is natural, and is written by correspondences, it is here said that "the third part of the trees and all green grass were burnt up," which means in the spiritual sense that all perception and knowledge of truth and good, as well as all true knowledge [scientificum] are destroyed, by these two corporeal, terrestrial, and merely natural loves.

[2] By true knowledge [scientifica] is meant all knowledge by which spiritual truth is confirmed, and which has life from spiritual good. For by knowledges [scientifica] a man may become wise or he may become insane. A man becomes wise by knowledges when he uses them to confirm the truths and goods of the church, which are spiritual truths and goods; and he becomes insane by knowledges when he uses them to invalidate and refute the truths and goods of the church. When they are used to confirm the truths and goods of the church they are called true knowledges, as also living knowledges; but when they are used to invalidate and refute the truths and goods of the church they are called false knowledges, and also dead knowledges. Knowledges [scientiae] are only means to uses, and they are such as the uses are that spring from them. They are living knowledges when man by means of them acquires for himself intelligence and wisdom. All intelligence and wisdom is from truths that are from heaven; such intelligence and wisdom, because it is from heaven, that is, from the Lord through heaven, is living, because it is the very spiritual life of man; but from falsities there can be no intelligence and wisdom, and if it is supposed to exist in anyone, yet it is dead, because it is from hell.

[3] This has been said to make known that "green grass" signifies true knowledge [scientificum], which is living, but "grass burnt up" signifies false knowledge, which is dead. When truth and good, which come from heaven, find no receptacle in the cognitions and knowledges with man, but evils and falsities which are from hell are received, then knowledges [scientifica] are not living but dead, and correspond to grass withered and burnt up. It is similar with man himself, for a man is such a man as the cognitions and knowledges are alive in him; for from living knowledges [scientiae] he has intelligence, but from knowledges not living he has no intelligence; and if they are dead in consequence of the confirmation of falsities by them there is insanity and folly.

[4] Such a man, from correspondence, is compared in the Word to "grass," and is also called "grass" in the following passages. In Isaiah:

The inhabitants have become as the herb of the field, as the greenness of the tender herb, as the grass of the housetops and as a field scorched before it is grown up (Isaiah 37:27; 2 Kings 19:26).

In David:

The wicked are cut down in haste as the grass, and wither as the greenness of the herb (Psalms 37:2).

In the same:

As for man, his days are as grass; as a flower of the field so he flourisheth (Psalms 103:15).

In the same:

The haters of Zion shall be as the grass of the housetops, which withereth before it is plucked up (Psalms 129:6).

In Isaiah:

The glory of Jehovah shall be revealed, and they shall see. The voice said, Cry; and he said, What shall I cry? All flesh is grass, and all its holiness is as the flower of the field; the grass hath dried up and the flower hath fallen off because the breath of Jehovah hath blown upon it. Surely the people is grass. The grass hath dried up, the flower hath fallen off; but the Word of our God shall stand up forever (Isaiah 40:5-8).

This is said of the Lord's coming, and of the revelation of Divine truth from Him at that time, which is meant by "the glory of Jehovah shall be revealed, and they shall see." That there will then be no true knowledge [scientificum] and no spiritual truth with men, is signified by "all flesh is grass, all its holiness is as the flower of the field; the grass hath dried up, the flower hath fallen off," "grass" meaning true knowledge, and "the flower of the field" spiritual truth. That man is such is meant by "all flesh is grass," and by "surely the people is grass; the grass is dried up;" "all flesh" meaning every man, and "people" those who are in truths, here those who are in falsities.

[5] In the same:

I am He that comforteth you; who art thou, that thou fearest man that dieth, and a son of man that is given for grass? (Isaiah 51:12).

These words signify that all things are from the Lord, and nothing from self-wisdom and self-intelligence. "Man" means man in respect to wisdom, and the "son of man" the same in respect to intelligence; that this latter is mere knowledge [scientifia] is meant by "is given for grass."

[6] In the same:

I will pour out My spirit upon thy seed, and My blessing upon thine offspring; and they shall spring up in the midst of the grass (Isaiah 44:3, 4).

The "spirit of Jehovah" signifies the Divine truth, and "blessing" signifies its multiplication and fructification; intelligence thence through true knowledges (scientifica) is signified by "springing up in the midst of the grass."

[7] In David:

Jehovah who causeth the grass to spring forth for the beast, and herb for the service of man (Psalms 104:14).

In the same:

Jehovah who prepareth rain for the earth, who maketh grass to spring forth upon the mountains, who giveth to the beast his food (Psalms 147:8, 9).

In Moses:

My doctrine shall flow down as the rain, My Word shall drop as the dew, as the mist on the grass, and as the drops on the herb (Deuteronomy 32:2).

In these passages "grass" signifies true knowledge [scientificum], and "the herb of the field" spiritual truth; for "the herb of the field" means what springs up in a field at first, that is, when it has just been plowed, therefore it is called "herb for the service of man." It is said "grass for the beast," and "as food for the beast," because "beast" signifies in the Word the affection of the natural man, and to this, true knowledge is for food and nourishment.

[8] In Job:

Behold the behemoth, which I made with thee; he eateth grass as an ox (Job 40:15).

"Behemoth" has the same meaning as "beast" in the Word, namely, the natural affections that belong to man, therefore it is said, "behold behemoth, which I have made with thee." His spiritual pasture is true knowledge; this is meant by "he eateth grass as an ox."

[9] That "green" signifies what is living can be seen without further explanation; for any vegetable subject while it is growing, that is, while it lives as it were, is green, but when it is no longer growing, or is as it were dying, its greenness perishes; therefore "green" or "to be green" signifies living or to be living; as can be seen also from the following passages (Jeremiah 11:16; 17:8; Ezekiel 17:24; 20:47; Hosea 14:8; Psalms 37:35; 52:8; 92:10) and elsewhere.