A small addition -
Saturn counts the number of breaths of a living being.
All planets must obey him in giving their results.

He is the fruits giver.
He is the Karma-Phal-Datha.
He evaluates karma and determines the fruits.
A planet must take Saturn's permission before giving a evil/good fruit.

Whenever one does penance for years and years, the native must have good Saturn position for
concentration and stability of mind.

Mercury is for learning and analytical power.
When one completes learning, treated as master, This status is ruled by Jupiter which is for wisdom.
When a master starts practicing there comes the role of Saturn.
Saturn is for applications of subject.
The next and final stage is ruled by Ketu, the moksha karaka, a head less planet,
all senses are absent including mind. Only heart works.

So the sequence is Mercury, Jupiter, Saturn, Ketu.

I wrote this post only to share knowledge.