The philosophy of Plotinus has the defect of encouraging men to look within rather than to look without: when we look within we see nous, which is divine, while when we look without we see the imperfections of the sensible world. This kind of subjectivity was a gradual growth; it is to be found in the doctrines of Protagoras, Socrates, and Plato, as well as in the Stoics and Epicureans. But at first it was only doctrinal, not temperamental; for a long time it failed to kill scientific curiosity. [...] Plotinus is both an end and a beginning-an end as regards the Greeks, a beginning as regards Christendom.