Page from the Codex Seraphinianus, completed by Luigi Serafini in 1978 and first published in two volumes in 1981. It is an illustrated encyclopedia of another universe, written in an alien script.
Ludwig Wittgenstein writes (Philosophical Investigations §269):
Let us remember that there are certain criteria in a man’s behaviour for the fact that he does not understand a word: that it means nothing to him, that he can do nothing with it. And criteria for his ‘thinking he understands’, attaching some meaning to the word, but not the right one. And, lastly, criteria for his understanding the word right. In the second case one might speak of a subjective understanding. And sounds which no one else understands but which I ‘appear to understand' might be called a “private language”.