We propose a universal readability index, GU, applicable to any alphabetical language and related to cognitive psychology, the theory of communication, phonics and linguistics. This index also considers readers’ short-term-memory processing capacity, here modeled by the word interval IP, namely, the number of words between two interpunctions. Any current readability formula does not consider Ip, but scatterplots of Ip versus a readability index show that texts with the same readability index can have very different Ip, ranging from 4 to 9, practically Miller’s range, which refers to 95% of readers. It is unlikely that IP has no impact on reading difficulty. The examples shown are taken from Italian and English Literatures, and from the translations of The New Testament in Latin and in contemporary languages. We also propose an extremely compact formula, relating the capacity of human short-term memory to the difficulty of reading a text. It should synthetically model human reading difficulty, a kind of “footprint” of humans. However, further experimental and multidisciplinary work is necessary to confirm our conjecture about the dependence of a readability index on a reader’s short-term-memory capacity.