He learns the same way we learn-by experience. Of course, he is at a disadvantage, having no parent there to guide him through his trials. But he slowly learns to "distinguish [his] sensations from each other": recognizing forms, light, sound, etc. He also learns to use tools, wielding fire to his advantage after burning himself once.
One day, when I was oppressed by cold, I found a fire which had been left by some wandering beggars, and was overcome with delight at the warmth I experienced from it. In my joy I thrust my hand into the live embers, but quickly drew it out again with a cry of pain. How strange, I thought, that the same cause should produce such opposite effects! I examined the materials of the fire, and to my joy found it to be composed of wood.
Essentially, he learns scientifically, through observation. These chapters represent a child taking his/her first steps in the world, slowly discerning each new sensation.