With the sole exception, perhaps, of Protagoras, who seems to argue against it, all serious thinkers before Aristotle made a sharp distinction between knowledge, real knowledge, certain truth (saphes, ale-theia ; later: episte-me-), which is divine and only accessible to the gods, and opinion (doxa), which mortals are able to possess, and is interpreted by Xenophanes as guesswork that could be improved.