Book excerpt: Stoicism is a logical philosophy. Herein, one may learn to reason like a Stoic, which leads to making progress toward living in accord with nature, and from which an abiding happiness is produced. Secundum Naturam is an exploration of Stoicism, given the thesis that the philosophical doctrines derive from Stoic logic. Itself, Stoic logic derives from one, first principle: contradiction does not exist in nature, although we contradict nature when we err. The connectives, modalities, and argument resolutions are all defined with respect to contradiction as conflict (between Both p and Not p). And when your will contradicts nature, you are living contrary to nature, while the goal is to live secundum naturam, according to nature. Best of all, learn how to improve yourself with Stoic logic, according to reason, according to nature, only with Secundum Naturam.