Seneca the Younger wrote the moral essay De Vita Beata ("On the Happy Life") to his brother Gallio. In a few words Seneca the Younger explains that the pursuit of happiness is the pursuit of the 'reason', reason meant not only using logic, but also understanding the processes of nature.
In Seneca the Younger's words, "rerum naturae adsentior; ab illa non deerrare et ad illius legem exemplumque formari sapientia est," which means "I assent to nature; it is common sense not to stray from it but to be molded according to its law and example."