The Alchemist understands that the elimination of untrue movements of the mind are the Key to the development of the Philosopher's Stone. Thus, the aim of the Alchemist is to expurgate untrue mentations in the Alchemical furnace. This is achieved by testing them in the external world and removing those which do not prove themselves to ring true internally and externally. When these thoughts are cleansed, the Alchemist completes the Great Work and embodies the Philospher's Stone.
The First Chapter of the Yoga Sutras echoes this Alchemical process.
1.1 Now, after much preparation, begins instruction in Union (yoga).
1.2. Union is controlling of the modifications (gross and subtle thought patterns) of the mind field.
1.3. Then the seer dwells in his own true nature, which is called Self-realization.
1.4. Otherwise he is of the same form as the thought-streams.
Alchemical Image by: https://resobscura.blogspot.com/2015/12/the-alchemy-of-madness.html
Yoga Sutra Words by: http://www.sacred-texts.com/hin/yogasutr.htm
Alchemical Words by: Iwanickimedia