This result sounds straight-forward enough, so can you prove it using Peano arithmetic? The first problem you encounter is that you cannot even state the result in Peano arithmetic — the language lacks the words you need to talk about arbitrary infinite collections of trees. However, if your infinite collection can be generated by some finite set of instructions, that is, if it can be generated by a computer, then Peano arithmetic can talk about it. Friedman showed that the sentence "in any infinite collection of finite trees that can be generated by a computer, some tree is contained in another" can be stated in Peano arithmetic, but that it can be neither proved or refuted.