The Twelve Operations · Operation 3 · Nigredo
Separatio Separation
In the vessel. The dissolved matter is sorted into its components, the heavy from the light. The Emerald Tablet says: separate the earth from the fire, the subtle from the gross.
In a life. Discernment after the flood: true from false, yours from inherited, essential from accidental. Not everything that was dissolved deserves to be put back together.
What in you was never yours?
The full section on separation, with the other eleven operations and the four stages, is in The Magnum Opus.