Bhagavad Gita Verse 19, Chapter 18

jnyaanam karma cha kartaa cha tridhaiva gunabhedataha |
prochyate gunasankhyaane yathaavacchrunu taanyapi || 19 ||
 
Knowledge and action and the doer are said to be of three types only in the science of gunaas, per the difference in gunaas. Listen properly to them.
 
jnyaanam : knowledge
karma : action
cha : and
kartaa : doer
cha : and
tridhaiva : three types
gunabhedataha : per difference in gunaas
prochyate : are said
gunasankhyaane : science of gunaas
yathaavat : properly
shrunu : listen
taani : them
api : only
 
A lot of ground was covered in the prior shloka, so let us recap. Jnyaanam or knowledge provides meaning to perception of an object, the jnyeyam. This perception, along with the meaning is provided to the jeeva, the individual soul, by the intellect. If the jeeva decides to be indifferent to this object, the matter ends there. If the jeeva decides to pursue that object, it commands the body to do whatever actions are necessary to obtain that object. While issuing the instructions, the jeeva becomes a doer or kartaa, and while experiencing the object, it becomes the experiencer of bhogtaa.
 
So why is this important? Each time we become the kartaa or the bhogtaa, we reinforce the notion that "I am the jeeva", and forget our true nature as the eternal essence, who is the saakshi or witness to all thoughts and action. The spiritual journey start from our present situation, and raises us higher step by step, away from our entanglement in action and experience, and closer towards our true nature as the unattached eternal essence. Shri Krishna says, do not worry, there is a way out. Anything that is asat, anything that is not the eternal essence, is in the realm of Prakriti. And we have seen in the fourteenth chapter that Prakriti is nothing but the play of the three gunaas. If we truly understand how the process of action works from the standpoint of gunaas, we can begin to extricate ourselves from its tendency to entangle us.
 
The great sage Kapila Muni created the Saankhya school of philosophy. The science of the gunaas is part of the Saankhya school. By analyzing the nature of the three main components of action, jnyaanam, karma and kartaa, which means knowledge, action and the doer, we can check whether they are sattvic, raajasic or taamasic as pertaining to us. This shloka begins a new section within the eighteenth chapter that categorizes these three components and some other related factors as demonstrating the quality one of the three gunaas. It is extremely practical and also summarizes many key concepts from the first five chapters of the Gita.