This is not really a foolish question. We all believe in an external world. But why should we believe that there is something like “matter” that exists? It is hard to say what matter is. In science we speak of matter and energy. I am speaking more generally than that. I am speaking of matter as any kind of ‘stuff’ that things are made of. We should believe in matter because we believe that some things can change into other things and all things that can change into other things can change into any thing eventually.
Remember that change requires something to remain the same. So everything that changes must have some aspect that remains the same. For example, if a tree is changed into firewood, then it is chopped into pieces. The tree and firewood are still made out of wood. Therefore, the stuff that they are made out of is the same stuff. If the fruit of a tree is planted, then the surrounding environment is used to add matter to the tree. But the tree and the surrounding matter are both made out of atoms. So it is in all cases. In order for one thing to become another thing, then they must share the same matter.
However, wood and atoms are different things. So when we say that everything is made out of matter, we do not mean that everything is made out of a particular kind of matter. So it must be possible for any kind of matter to be transformed through some means or other into any other kind of matter. At least this must be true if all kinds of matter are kinds of one thing rather than many things. In science we know that this is true. Any form of matter can become energy, and any form of energy can become matter. Furthermore, all forms of energy can be transformed into heat energy. All forms of matter can also become heat energy. Therefore, there is a good scientific reason to believe this. Experience in general also tells us that this is true. We know that living things grow from the environment and return to that environment when they die. So it is with all other things as well. So there is every reason to think that this is true.
This means that all things that can change into something else are made out of matter. If immaterial things exist, then they cannot change into another kind of thing. Remember that change requires that something remain the same. So if what something is changes, then the stuff that it is made out of must not change. But immaterial things are not made out of any kind of stuff at all. Therefore, they cannot change into another kind of thing. But material things must exist because some things do change into other things. Therefore, matter exists.