The thirteenth chapter introduces a pair of terms and then spends its length on them. The body is called the field, and the one who is aware of it is the knower of the field. Krishna adds that he is the knower of the field in every field, and that knowing the two together, and their difference, is what he considers to be knowledge.
The field is then itemised, and the list is unexpectedly long: the elements, the sense of self, the understanding, the unmanifest, the eleven senses, the five objects of sense, and then desire, aversion, pleasure, pain, the organism, awareness and firmness. Emotions and states of mind are placed on the field side, among the things observed, rather than on the side of the observer. That single move is what the chapter is for. What most people take to be their inmost self, their wanting and disliking, is being classified with the body.
Knowledge is then defined, and the definition is a surprise. Instead of propositions to be believed it gives a list of qualities: absence of pride and of pretension, non-injury, patience, uprightness, service of a teacher, purity, steadiness, self control, dispassion toward the objects of sense, absence of egotism, the perception of the evils in birth, death, old age and sickness, non-attachment, evenness of mind in the desirable and the undesirable, unswerving devotion, resort to solitary places, and dislike of crowds. Everything else, the chapter says, is ignorance.
Defining knowledge as a character rather than as a content is unusual enough to be worth pausing on, and it explains why the Gita can treat an uneducated devotee as knowing more than a learned one.