512. And the third part of the sea became blood, signifies that in consequence everything therein became the falsity of evil. This is evident from the signification of the "third part," as being all (See above, n. 506); also from the signification of the "sea," as being the natural man (See just above, n. 511); therefore "the third part of the sea" signifies the whole natural man and everything therein; also from the signification of "blood," as being falsity of evil (See also above, n. 329). From this the spiritual sense of this verse can be seen, namely, that "the great mountain burning with fire was cast into the sea, and the third part of the sea became blood," signifies that the love of self, when it enters into and occupies the natural man, turns every knowledge [scientificum] into the falsity of evil.

[2] The love of self is a love purely corporeal, springing from the rising up and fermenting of worn out parts and the titillation caused by these inwardly in the body; in consequence of which the perceptive faculty of the mind, which requires a pure atmosphere, not only becomes dull and gross, but even perishes. That this is the origin of the love of self can be seen from its correspondence with human dung; for those who have been corroded by this love, when they come into the other life love above all things stercoraceous filth, and its stench is grateful to them, which proves that the effluvium therefrom pleasantly affects the sensory of their smell, as it had before affected the general sensory, which is extended by interior cuticles in every direction. From this alone it can be seen that the love of self is more gross and foully corporeal than any other love, and consequently that it takes away all spiritual perception, which is a perception of the truth and good of heaven and the church. Moreover, it shuts up the spiritual mind and fixes its seat entirely in the natural and sensual man, which communicates most closely with the body and has no communication with heaven. From this again it comes to pass that all those in whom the love of self is dominant are sensual, and do not see the things that belong to heaven and the church except in the densest darkness; and furthermore they reject and deny these whenever they are alone and are thinking with themselves. From this the signification of "the third part of the sea became blood, in consequence of the great mountain burning with fire that was cast into it," can now be seen.

The number "four" in the Bible represents things being linked together or joined. This is partly because four is two times two, and two represents the ultimate linking between our desire to be good and our understanding of truth. We can also see this in the fact that most buildings are rectangular, with the four sides linking together to make a whole. We also divide directions into four -- north, east, south and west -- and talk about the "four corners" of something meaning all of it. So our special thinking naturally looks at four sides as linking together into a whole.

(Referencat: Apocalypse Explained 417; Arcana Coelestia 1686, 9103, 9601, 9767, 9864)