There is a sense of “prove” which implies certainty. If your friend is using this sense he is right- you cannot be certain that people stop existing when you stop perceiving them. In fact, he is not being nearly skeptical enough. You can’t be certain people people exist when you’re perceiving them either! Your senses could be lying to you- in fact people often dream or hallucinate people who aren’t really there.
But what we’re doing when we say something exists is making a model to explain our sense perceptions. There are models consistent with our senses that do not involve people existing—but they are bad models. Believing that all of reality is structured around your perception and that things come into and out of existence based on whether or not you sense them is not a very good model of the world. In fact it is a backwards and totally unhelpful model of the world. The point in saying “My mother exists” is to explain why you have all these sense perceptions: her image, the sound of her voice, memories of her, etc. The best way to explain and predict these perceptions is by thinking of your mother as an independently existing entity… “I saw my mother at 5:00 because that is when she gets home from work I didn’t see her before then because she was at work.” (note, one feature of hallucinations is that they usually don’t lend themselves to being modeled as externally existing entities). And this isn’t just true of people: it is true of all your beliefs.
There is a sense of “prove” which implies certainty. If your friend is using this sense he is right- you cannot be certain that people stop existing when you stop perceiving them. In fact, he is not being nearly skeptical enough. You can’t be certain people people exist when you’re perceiving them either! Your senses could be lying to you- in fact people often dream or hallucinate people who aren’t really there.
But what we’re doing when we say something exists is making a model to explain our sense perceptions. There are models consistent with our senses that do not involve people existing—but they are bad models. Believing that all of reality is structured around your perception and that things come into and out of existence based on whether or not you sense them is not a very good model of the world. In fact it is a backwards and totally unhelpful model of the world. The point in saying “My mother exists” is to explain why you have all these sense perceptions: her image, the sound of her voice, memories of her, etc. The best way to explain and predict these perceptions is by thinking of your mother as an independently existing entity… “I saw my mother at 5:00 because that is when she gets home from work I didn’t see her before then because she was at work.” (note, one feature of hallucinations is that they usually don’t lend themselves to being modeled as externally existing entities). And this isn’t just true of people: it is true of all your beliefs.