Verse 2.23: The soul can never be cut to pieces by any weapon, nor burned by fire, nor moistened by water, nor withered by the wind.
Knowledge of both the material and spiritual are given in different ways throughout the Gita. Knowledge is given in a way direct way (i.e. we are not this body but the spirit soul), it is given in extremely detailed ways (as we will come to see) and here we are seeing knowledge given to us in another way - through negation.
As spiritual beings who have undergone lifetimes of experiences, it can be very difficult to understand subject matters that cannot be explained through the material senses. So what does Krsna do? In order to help us understand what the soul is, Krsna is putting it in context of the material senses. That is, he is describing the existence of the spiritual by explaining what it is not.
We all have experience of this. Sometimes when we are trying to explain a concept or object to someone, we define it by "what it is not." That is, if we cannot express what it is, then we express it in a way that describes what it isn't.
Since the spirit soul is not material, but due to the fact we can only relate with what is material, this explanation through negation is being utilized. That's not to say that Krsna hasn't tried to explain what the soul is in a detailed way. As a reminder, in previous verses, Krsna explained that the soul (by the symptom of consciousness) pervades the entire body, is eternal and is infinitesimally small.
Here, however, more detail is being given. The soul cannot be cut to pieces, be burned, be moistened by water nor withered by the wind. All the material elements cannot effect the soul. That's how powerful and protected the spirit soul is.
God is the perfect teacher. He knows the advancement and level of his students and thus teaches accordingly. We are so lucky to have a Lord who loves us so much!