This doubt is based on the experiences and beliefs of others that provided evidence for the piano lessons. How do we know that other minds even exist in the external world? The boy is depending on the memories of other minds, which is questionable. If we are uncertain about our own past experiences and existence, how can we be sure about anyone else's, especially if there is a question about the possibility of an external world? .
3. Does skepticism about the external world require skepticism about the past? I believe it does. As I have stated before, the inside of your own mind is the only thing you can be sure of. Whatever you believe is based on your experiences and thoughts, feelings and impressions. But, the argument that there must be an external physical world because you are able to visualize everything around you has been called into question. Remember, everything that you know comes from the contents of your mind, and there is no possible way for us to be sure that there is anything actually going on outside of our mind. So, if we can't be sure of real things existing in the physical world, how can we possibly be sure of events that happened to us, or for that matter, anyone else in the past? The example of the boy questioning whether he took piano lessons previously can be used again. Since the boy may or may not even exist in the external world (the skeptic's view), how could he possible rely upon past memories and experiences that may or may not have occurred? If all the boy has to go on is the contents of his mind, which may not exist in the physical world, then there is a reasonable level of doubt that he may never had taken piano lessons before, as all of his past experiences and memories may just be figments of his imagination. In order to be skeptical about the external world, skepticism about the past must be required because if you are questioning what is real outside your mind, the question of all things past will inevitably evolve.
