Well if you can't concede that anything is real, that sort of makes you crazy doesn't it? A tree is real. But the concept of a tree and the word "tree" and all the ideas you have about the tree and what tree means, is that real? No, because it doesn't change the nature of the tree. When you cease to exist, the tree will still be there. Can you be absolutely 100% sure of that? Also no. But if you believe that other people are conscious individuals like you are and that some of them die and the tree keeps going, you can concede that it is probably true that the tree exists separate from your idea of it.