For Socrates, philosophy was the sole authority for judging what kind of life is best for a human being and how we should implement that philosophical analysis in our own particular lives. But, according to Socrates, a philosophical life is more than just following a set of principles based on good reasons. Philosophical exploration and the constant testing of philosophical claims is essential to a philosophical life, and that continuous process of reasoning provides the motivations for how one lives. Philosophical argument and analysis, as pervasive, on-going activities, are the central features of the best life for human beings.
Aristotle, for the most part, followed Socrates on this. Philosophy is the authority on how one should live, and the best life for a human being is one in which a philosophical investigation into the good life is pervasive and directs one’s practical activity. But for Aristotle, such a life can be lived only by persons dedicated to philosophical theory as a central occupation. For virtuous political leaders or other citizens, a commitment to leading a contemplative life of philosophical study, beyond one’s formative years, is optional. One can lead a good life, although perhaps not the best life, by relying on the sound philosophical reasons of others.
Thus, Aristotle begins the process of separating philosophy as a way of life from the formal study of philosophy.
The Stoics then pushed this separation further. For them, philosophy is also the sole authority on the best life because it provides the proper conceptual framework and motivations for leading such a life. But one can accomplish this by following precepts and using practical exercises to control emotions and desires. The formal practice of philosophical reasoning as on on-going occupation is fully optional. On relies on stoic sages to do the philosophical work.
This trajectory strikes me as a mistake.
To my mind, philosophy is an activity, not a set of principles, and it’s an activity that involves an ongoing search for truth through a process of reasoning. If philosophy is a way of life, it is so because one’s life internalizes a process of reasoning that is distinctly philosophical. However, it remains problematic to say precisely what that process of reasoning looks like in a way that comprehensively captures what philosophers have been up to throughout philosophical history.
Philosophy: A Way of Life 