Bhagavad Gita Verse 18, Chapter 18

jnyaanam jneyam parijnyaataa trividhaa karmachodanaa |
karanam karma karteti trividhihi karmasangraha || 18 ||
 
Knowledge, the known and the knower, these three initiate action. The instruments, the target of action and the doer, these three are the basis of action.
 
jnyaanam : knowledge
jneyam : known
parijnyaataa : knower
trividhaa : three
karmachodanaa : initiate action
karanam : instruments
karma : target of action
kartaa : doer
iti : these
trividhihi : three
karmasangraha : basis of action
 
The culmination of the Gita teaching is the realization that our true nature is the eternal essence, that does not act in this world, nor experiences anything in this world, since action and experience are in the realm of Prakriti. Most of us, however, are still becoming qualified for this teaching through the practice of karma yoga. Shri Krishna recognizes the need to give an in depth analysis of action for people like us. To that end, he uses this shloka to begin that topic. Each word, each term needs to be understood clearly, since the regular meanings may confuse us.
 
So then, what is the genesis, the birth of an action? How does an action commence? Our sense organs send a report to the mind of having seen, felt, touched, tasted or smelled something. This is the process of perception. Or, a thought about a prior perception arises in the mind. Both these processes are the same for all people in this world. Both you and I perceive a red apple in the same manner. In this case, the red apple is termed as jneyam, an object which is known. This is step one.
 
What happens next? Both of us see the same red apple, but you may love it, someone else may hate it, and I would be indifferent to it. This difference in our view towards the apple, our worldview in general, is due to the difference in our samskaaras. We attach a certain meaning to objects, people and situations based on our samskaaras. This individual vision of the world is termed knowledge or jnyaanam. It looks at the object in question and generates a sense of attraction, repulsion or indifference. This is step two.
 
Next, this notion of attraction, repulsion or indifference creates a modification in the intellect called the "doer", the kartaa. It is a phantom, illusory entity which says "I want the red apple, go get it", or "I hate this red apple, throw it away". The doer issues these instructions to the karana, the organs of action, which then do as they are told. The action of grasping the apple or throwing it away is carried out. Note the the doer only comes into existence when there is attraction or repulsion. The jeeva is a witness, indifferent to likes or dislikes. So the doer, the organs of action and the object make up karma sangraha, the basis of action. This is step three.
 
When finally, the object, the target of action is consumed by the senses, another modification of the mind called the "enjoyer", the bhoktaa, arises. It creates the notion that "I have experienced this object, and it gave me joy/sorrow". This is the parijnyaata, the knower, mentioned in the shloka. Furthermore, a record of this experience, whether pleasurable or painful, is stored in the unconscious aspect of our personality, the causal body. This record, this samskaara, becomes the seed of future action by creating thoughts of desire in the mind, prompting further actions and experiences. This process of enjoyment of an object is the fourth step.
 
So these four steps taken together describe the lifecycle of an action, from start to finish.

Bhagavad Gita Verse 2, Chapter 17

Shree Bhagavaan uvaacha:
trividhaa bhavati shraddhaa dehinaam saa svabhaavajaa |
saatvikee raajasee chaiva taamasee cheti taam shrunu || 2 ||
 
Shree Bhagavaan said:
Threefold is the faith that comes from the nature of the embodied ones, saatvic, raajasic and taamasic. Listen about that from me.
 
trividhaa : three fold
bhavati : is
shraddhaa : faith
dehinaam : embodied ones
saa : they
svabhaavajaa : come from nature
saatvikee : saatvic
raajasee : raajasic
cha : and
eva : also
taamasee : taamasic
cha : and
iti : in this manner
taam : about that
shrunu : listen
 
Arjuna began this chapter by posing the following question. If one does not have access to the scriptures but uses their faith to guide their lives, what kind of a position is that? Shri Krishna does not answer that question directly. The entire chapter, in fact, is the answer to that question.
 
It is not an easy question to answer because faith differs from person to person. We can say that we trust our faith, our conscience, to guide us towards performing right actions. But the faith of a terrorist is different than faith of a saint. If we do not have guru to guide us, we cannot assess our faith objectively. So how should we tackle this issue?
 
Shri Krishna gives us a starting point to help us. We first need to understand what he means when he says that faith comes from svabhaava or nature. What exactly is meant by nature here? Nature refers to the collection of samskaaraas or impressions that are present in a person, a “dehin” or embodied one, from birth until death. Whenever we perform an action and get its result, both the action and result are ingrained in our mind in the form of an impression. This impression is called as samskaara. Whenever we repeat the action and get the same result, the impression is further ingrained and strengthened. If we repeat it often enough, it becomes a habit. It could be a good habit such as respecting our elders, or a bad habit such as smoking.
 
So this nature, this collection of samskaaraas or impressions, is where our faith comes from. Now let’s go back to Arjuna’s question. In the absence of scriptures, in the absence of a rational, logical means to check our actions, what will happen if we use our faith instead? If someone swears loudly at us in the middle of the street, we will respond instinctively at that point rather than using logic or reason. We may punch the person who yelled at us, or we may ignore him. It depends on the makeup of our nature.
 
Now, based on the makeup of our nature, we can classify our faith into three types: saatvic, raajasic and taamasic. But it is difficult to do so directly. We will have to infer the nature of faith through something else. Shri Krishna tells Arjuna to listen, since he will explain how to do this.