Stoicism is a philosophy of unity and coherence. All of its parts fits together beautifully to help us cope with our own thoughts and emotions, with other people, and with the greater world around us. Let's look in detail at how the Stoic goal of living in agreement can guide us toward a good and fulfilling life.
Coherence and Virtue
One of the defining doctrines of Stoicism is that virtue is the only true good in life. (Other "goods" such as health, wealth, and external success are considered preferred indifferents but are not good in the sense that virtue, or excellence of character, is good.) But what justification is there for such a counterintuitive idea? Stoics don't start with the idea that virtue is the only good--it's not an article of faith--but end up there after carefully observing and thinking about the world. Zeno, the founder of ancient Greek Stoicism, put it like this:
Zeno described one’s aim as follows: “living in agreement”—that is, living according to a rationale which is single and in agreement, on the grounds that those who live in conflict are unhappy. His successors, articulating this further, produced “living in agreement with nature,” taking what Zeno said to be an incomplete predicate. Cleanthes, his first successor as head of the school, added “with nature,” and he gave the following definition: “one’s aim is living in agreement with nature.” Chrysippus, wishing to make this clearer, expanded as follows: “living in accordance with experience of what comes about by nature.” (Arius Didymus, cited in Annas, 2003)
Zeno, apparently, also used the expression "living in agreement with nature" and equated both these formulations to "living in agreement with virtue" (Diogenes Laertius, 7.87). This pithy formula has multiple layers of meaning and has been interpreted and re-interpreted down through the ages. But let's focus for a moment on Zeno's original sense of the expression. Living in agreement means “living according to a rationale which is single and in agreement, on the grounds that those who live in conflict are unhappy.” We are not necessarily concerned with devising an externally coherent theoretical system, no matter how beautifully constructed that system may be, but rather seeking internal coherence. In other words, living a coherent life as a coherent person, so that all your thoughts, actions, and desires align on a single plane, both now and at all times in the future. Of course we do want our principles to form a coherent system, but that’s not enough. We also want to live in coherence with those principles. As mortal beings we have no time or energy to waste. We need to make sure every thought or action counts toward our final purpose.
But if we’re going to be that committed to coherence, we need to make sure we are working toward the right thing. So what endgoal in life could possibly be worth that much devotion? What organizing principle could there possibly be that allows us to live in agreement and thereby to become happy and fulfilled? You guessed it: virtue. The pursuit of virtue—arete, excellence of character, nobility of mind and spirit, skill in the art of being human—is a conceptual ideal that can consistently direct our thoughts and actions in a way that benefits us and others around us. To understand why this should be so, we need to think carefully about what it means to be human.
The defining characteristics of humans, those that set us apart from other animals and enable us to live as humans do, are our extreme sociability and our advanced cognitive capacities. Without these two defining features, we certainly wouldn't be the same species we are today. So someone who is the most skilled at being a human must be the most skilled in using these two capacities: living as part of a group and understanding the world accurately.
If we imagine the sage as the most exemplary human in the world--i.e., the person most skilled at being human--we think of someone who is always correct in her thoughts and actions toward others, someone with an accurate and insightful understanding of the ways of the world and her role in it. In other words, this paragon of humanity would be wise, just, courageous, and self-controlled in every situation she encountered in life. Her thoughts and actions would consistently align with the goal of living as an exemplary human would; she would be perfectly virtuous because she is perfectly coherent. All her motivations, opinions, and desires would be aligned in service to a single goal (virtue), and all her judgments would have the same source and basis (virtue). She would not be omniscient, of course, because she is only human. But given accurate information, her judgment would be absolutely correct because the source of her judgments would be absolutely correct. She wouldn't know everything, but she would know how to respond appropriately to everything. That is the infallible rationality of the sage: not perfect knowledge, but perfect judgment.
Even though the sage doesn't actually exist, we need the ideal of the sage because, being human, we cannot easily separate virtue as such from virtue as embodied in an individual person. We can and do have a concept of virtue in the abstract, but that concept doesn’t fully come to life until it’s applied in the context of a virtuous disposition, a virtuous choice, a virtuous action. That’s why when we think of virtue, we think of specific people who display virtue and the specific ways in which they are virtuous. We strive to become virtuous, not to possess an object called virtue. Virtue differs in kind from external goals because it is truly interior to our selves; it does not depend on anything else.
But, although we may not hope to actually reach it, virtue is not merely an idle abstraction. Humans are instinctively primed to develop toward virtue with appropriate education and training, even if we never become sages. The brilliant, fascinating ancient Stoic theory of oikeiosis suggests that we all have the seeds of virtue within us. (For more on oikeiosis, see Development.) This process can and does go wrong when people are deceived into thinking externals are goods by the persuasiveness of external appearances and the conventional morality taught by their families and society (Diogenes Laertius, 7.89). But the potential for virtue is embedded in our nature, and with the right training, we can come ever closer to realizing our full potential.
Relationships: Logic, Ethics, Physics
We've seen that living in agreement = living in agreement with nature = living in agreement with virtue, because they all amount to the same thing: becoming the sort of excellent person that humans, by our nature, are meant to be. When you make this your endgoal in life, you eliminate inner conflict and self-reproach and become a coherent person whose motivations are always toward the good. Your life is imbued with meaning, no matter what happens to you externally, because you are consistently engaged in the process of being the sort of person you ought to be.
So much for the broad outline of virtue. How do we actually go about living in agreement? We can start by breaking down our experience of the world into components. The ancient Stoics discussed their philosophy in terms of three parts: logic, ethics, and physics. These three disciplines mediate our relationship to ourselves (logic), our relationship with other people (ethics), and our relationship to the world at large (physics). Talking about philosophical theory is not the same as living our philosophy, but it gives us a starting point to organize these ideas in our heads. After we understand the theory, we can make sure we are living up to our ideals in daily life.
Logic helps us make sense of the world by accurately evaluating our sensory experiences and organizing them through inner discourse (Hadot, 2001). Here we can look to another dimension of living in agreement, which Chrysippus calls living in accordance with experience of what comes about by nature. In my interpretation, this means we rely on our experience as humans to make sense of the world. We use our natural human faculties and capacities (things like eyesight, sense of touch, reasoning ability, creativity, sense of wonder) to take in information about what is happening around us, detect patterns in that information, reflect on what we discover, and try to understand what it means. The ancient Stoics defended the position, against Skeptical attack, that sensory experience is a reasonable basis for deriving knowledge about the world. All normal humans gain experience in this way. It’s how we learn about the world, fit into our societies, and decide (consciously or unconsciously) what to do. It’s how our brains work. Experience is the basis for our lives, so it should also be the basis for our philosophies of life.
But that doesn’t mean we should rely on unquestioned or unreflective experience. We have to learn how to reflect on our experiences in a new way and detect new patterns in the input; we need to change our perspective. This is where Stoic psychology is invaluable. The Stoic theory of impressions distinguishes raw sensory information from our interpretation of that information. All too often people believe their impressions correspond to raw facts about the world, when (in fact) this is not true at all. Our brains automatically add an interpretation of what an external event means for us, but this interpretive step is so quick and automatic that we don't realize it's happening. Stoic logic enables us to step back and separate the objective information from our subjective interpretation. Our ability to analyze and rationally manipulate our own inner experiences is crucial to gaining an accurate understanding of the world. Everything else that we might want to do in a philosophical life depends on this capacity to objectively manipulate our inner discourse.
Ethics, meanwhile, requires us to apply this same objectivity to our interactions with other people. Stoicism addresses one of the central problems facing any ethical system, namely, helping us get over ourselves. When we learn to see things objectively, we realize that we are part of a larger whole (humankind), and we must therefore act in a way that benefits the whole group. Through the developmental process of oikeiosis, we learn how to act appropriately toward others, moving toward the Stoic ideal of justice.
Our ability to live in agreement, therefore, is tightly linked to the development of our relationship with others, as well as development of our relationship with ourselves (i.e., re-categorizing our life priorities and modifying our response to impressions). But we are also physical creatures living in a biological world. If we want to live coherently and virtuously, we must learn to cope maximally well with the world around us. This is where Stoic physics comes in.
Physics mediates our relationship with the world at large, particularly the non-social natural world, by ensuring that our responses to and interactions with nature are consistent with our pursuit of virtue. For our purposes here, I distinguish between the humanist Stoicism that I practice from any ancient or modern form of Stoicism that sees the world as directed by a providential intelligence.
The scope of a humanist form of Stoic physics is quite different from the scope of ancient Stoic physics. While ancient physics had a very broad purview, including theology and what we would today consider sciences like anatomy and astronomy, this is no longer required by contemporary Stoicism. We now have other ways of answering questions about the origin and nature of the physical world, since the hard sciences have branched off and developed new methods of inquiry. That's why a humanist account of Stoic physics only answers the question how do we live in agreement with nature? We don’t need to look for answers to questions such as how did the world begin? or what is the nature of matter?—not because those are bad questions, but because the philosophy is not designed to do that. Today we don’t need our philosophy of life to provide an account of the origin or nature of the physical world. What we need is a coherent way of living in agreement with it.
However, just because physics has a narrower scope within a humanist Stoic framework does not mean it is less important. On the contrary, it is as essential as ever. Physics tells us what our attitude should be toward nature: reverent, grateful, respectful, full of awe and wonder at the ineffable mystery of the universe. You can develop this attitude regardless of whether you think the universe is animated by a divine principle or not. And considering the degraded state of our natural world in the 21st century, I think this attitude is more important than ever for us to cultivate.
An attitude of reverence and awe toward the natural world goes hand in hand with our understanding of our relative place in it, just as a proper attitude toward the social world goes hand in hand with our understanding of our relative place in it. If we think we are more important than other people, or we don’t stop to consider the big picture, we will never become virtuous or live a coherent life. This is why Stoic physics, ethics, and logic cohere together so tightly. In order to live a happy and fulfilling life, we must learn how to see objectively; and by learning how to see objectively, we understand our proper role in the social and natural world. All three parts of Stoic philosophy support and reinforce each other. Without one, the others don't make as much sense.
Returning to our ideal of living in agreement, we can see how Stoicism, as a philosophy of life, both relies on and supports the coherence of human nature. It helps us understand ourselves and our environments, living consistently across time and all situations. We are finally able to focus our energy on a single goal that benefits ourselves and others, enabling us to become the type of person we want to become.
 For a very interesting theory on how our social nature shapes our other advanced cognitive abilities (language, abstract mental representations, cooperative planning, etc.), I highly recommend Michael Tomasello’s Becoming Human: A Theory of Ontogeny, Harvard University Press, 2019.
 For an in-depth description of oikeiosis, see Annas (1993) and Graver (2009).
 If you're the sort of person who is interested in the is/ought gap, see Annas (1993) for a discussion of how this is not relevant to ancient ethics.
 There are many excellent books on ancient Stoic cosmology and theology. For more information, see Sellars (2006) or Hadot (2001).
Annas, J. (1993). The Morality of Happiness. Oxford University Press: Oxford.
Diogenes Laertius, Lives of Eminent Philosophers. Cited in Annas, J. (1993). The Morality of Happiness. Oxford University Press: Oxford.
Graver, M. (2009). Stoicism and Emotion. University of Chicago Press: Chicago.
Hadot, P. (2001). The Inner Citadel: The Meditations of Marcus Aurelius, transl. Michael Chase. Harvard University Press: Cambridge.
Sellars, J. (2006). Stoicism. University of California Press: Berkeley.