This is a good question. Babies are certainly developing rapidly in the first few months and learning all the time. We might not think so, because they cannot express what they learn to us, but this does not mean that they are not actively engaging their new world.
According to the Mayo Clinic, babies are able to develop motor skills in the first few months, such as move their necks, arms and legs. So, they are discovering motor abilities. Babies are also learning to hear. By the first few weeks, they can respond to noises. In addition, babies in the few month will be able to see better and even focus on your face and even intentionally respond with a smile, which is amazing if you think about it. Finally, within the first few months, babies are able to communicate with you with coos, if you talk to them gently. So, in a word, children are learning and discovering right from the beginning.