Arcana Coelestia # 9861
9861. 'It shall be square when doubled' means that which is righteous and perfect. This is clear from the meaning of 'square' as that which is righteous, dealt with in 9717. The reason why that which is also perfect is meant is that the breastplate was doubled, and that which is doubled implies all aspects of good and all aspects of truth. What is on the right side implies the good from which truth springs, and what is on the left implies the truth springing from good, 9495, 9604 (end), 9736, thus also the two perfectly joined to each other. This accounts furthermore for the meaning of 'two' as things joined together, 8423, also every single aspect of them, 9166, and their completeness as well, 9103.
