panchaimaani mahaabaaho kaaranaani nibodha me |
saankhye kritaante proktaani siddhaye sarvakarmanaam || 13 ||

Learn these five factors for the accomplishment of all actions, O mighty armed, which are spoken of in the Saankhya in which actions culminate.
pancha : five
imaani : these
mahaabaaho : O mighty armed
kaaranaani : factors
nibodha : learn
me : from me
saankhye : Saankhya
kritaante : culmination of actions
proktaani : said
siddhaye : accomplishment
sarvakarmanaam : all actions
Shri Krishna begins describing the true nature of action with this shloka. Typically we tend to think we are responsible for initiating, executing and completing every action, from the simplest action like drinking a cup of tea, to a complex action like planning the construction of a 100 storey building. He says that there are other factors are play which are responsible for the accomplishment of all actions, as denoted by the Saankhya or Vedaanta. He also conveys to Arjuna that a new topic has started, by calling out his name.
In the fourth chapter, we came across a shloka which stated that all actions in their totality culminate in knowledge. There are two components to this knowledge. The first is that the self, the eternal essence, the aatmaa, is actionless, since there is no possibility of change or modification in something that is changeless. This leads us to the second component of knowledge, which is as follows. If I, the self, am not performing action, something else must be doing so. As long as we are not fully convinced that something else is performing actions, we will hold on to the notion that we are doing so.
A naive person sits in a bus and thinks that he is driving it. You have to convince him that he is not driving it, but it is the bus driver that is driving it. In the same way, Shri Krishna gives us a detailed analysis of action and its components, such that we may come to the right conclusion. We are naive in thinking that the I, the self performs action when the Saankhya, the Vedaanta tells us that we do not. Once we come to this conclusion, we will automatically renounce the doership of action, and consequently, free ourselves from the chain of action and reaction.