Seneca wrote a number of books on Stoicism, mostly on ethics, with one work ("Naturales Quaestiones") on the physical world. Seneca built on the writings of many of the earlier Stoics: he often mentions Zeno, Cleanthes, and Chrysippus; and frequently cites Posidonius, with whom Seneca shared an interest in natural phenomena. He frequently quotes Epicurus, especially in his "Letters". His interest in Epicurus is mainly limited to using him as a source of ethical maxims. Likewise Seneca shows some interest in Platonist metaphysics, but never with any clear commitment. His moral essays are based on Stoic doctrines. Stoicism was a popular philosophy in this period, and many upper-class Romans found in it a guiding ethical framework for political involvement. It was once popular to regard Seneca as being very eclectic in his Stoicism, but modern scholarship views him as a fairly orthodox Stoic, albeit a free-minded one.