What is the self? As with any complex and slippery concept, we can draw on literally thousands of years worth of philosophical discussions in both the eastern and western traditions to address the question. But perhaps it will be simpler to begin with two contrasting definitions from a dictionary:

In philosophy:

The ego; that which knows, remembers, desires, suffers, etc., as contrasted with that known, remembered, etc.

The uniting principle, as a soul, underlying all subjective experience.

Setting aside metaphysically questionable talk of "soul," it should be obvious that there is such thing as the self. We do "know, remember, desire, suffer, etc.," and we do have unified subjective experiences. Whatever turns out to make that possible is the self.

This, however, doesn't mean that the self is a stable entity, or a specific thing located somewhere inside our brain. Nor, certainly, is there any reason to believe that -- whatever it is -- it will survive our physical demise.

The most "skeptical" conception of the self in western literature is usually attributed to David Hume. In his A Treatise of Human Nature he famously wrote: "I may venture to affirm of the rest of mankind, that they are nothing but a bundle or collection of different perceptions, which succeed each other with inconceivable rapidity, and are in a perpetual flux and movement."

The Stoic take on what constitutes the self is eminently practical and - when properly understood - can dramatically change our life.

This turns out to be analogous to the oft-mentioned Buddhist notion of anatta, or no-self. Contra popular understanding, however, this is not a statement that there is no self, but more precisely that there are no essential or unchanging things that constitute our selves.

By contrast, the Stoics are often presented as putting forth an essential conception of the self. I do not think this is correct, as the Stoic notion is actually similar to the Buddhist one as well as the one put forth by Hume. Moreover, the Stoic take on what constitutes the self is eminently practical and -- when properly understood -- can dramatically change our life. For the better.

To explore the issue, let us consider two prominent figures that have shaped Stoic thinking and continue to do so: the Pre-Socratic philosopher Heraclitus, and the early second century Stoic philosopher Epictetus.

Heraclitus was a major influence on the Stoics, who imported into their philosophy a number of key elements from him. One such element is encapsulated by the Heraclitean dictum that "We never step into the same river twice." Why not? Because both the river and ourselves are never the same. This is because everything in the universe is a process, not a static thing. Even objects that appear to be static on a human timescale, such as mountains, are in fact a snapshot of a series of dynamic processes, such as tectonic movements and erosion.

The Stoic philosopher-emperor Marcus Aurelius often deployed Heraclitean metaphysics in his Meditations, drawing out its ethical implications: "Acquire the contemplative way of seeing how all things change into one another, and constantly attend to it, and exercise yourself about this part of philosophy. For nothing is so much adapted to produce magnanimity." Contemplating the impermanence of everything, including our own self, leads to magnanimity, literally "greatness of soul," because one is less likely to be attached to specific configurations of events and more likely to accept change as an inherent and necessary aspect of how the cosmos works. Buddhists would nod approvingly.

And this brings us to Epictetus. A core aspect of his brand of Stoicism is that by far the most important thing we can do in our life is to improve our prohairesis, a Greek word often translated as volition, but that more broadly indicates our ability to arrive at judgments about things and events. Why would our prohairesis be so important? Because without it we wouldn't be human at all, and because when it doesn't work properly we are bound to make all sorts of ill-advised decisions, thus making our life worse.

Our faculty of judgment allows us to act "as nature prescribes," which for a Stoic means reasonably and pro-socially.

In his Handbook for a good life, Epictetus says: "What quality belongs to you? The intelligent use of impressions. If you use impressions as nature prescribes, go ahead and indulge your pride, because then you will be celebrating a quality distinctly your own." An "impression" is our preliminary assessment of whatever we perceive. For instance, I may see a gelato stand down the street and have the impression that it would be good for me to get a three-scoop cone. However, my prohairesis kicks in, challenging the impression, and reminding me that the gelato will spoil both my dinner and my waist line. So I forgo the gelato. This time.

Notice that Epictetus says that our faculty of judgment allows us to act "as nature prescribes," which for a Stoic means reasonably and pro-socially, because -- as modern comparative primatology confirms -- our nature is that of social, cooperative animals capable of reason. He also says that we can "indulge our pride" in the good functioning of our prohairesis, because it is the most distinctly human and valuable faculty we possess. In the Discourses he elaborates on why this is so crucial for our wellbeing:

"See that you do not act like a sheep, or else again the Human in you perishes. You ask how we act like sheep? When we consult the belly, or our passions, when our actions are random or dirty or inconsiderate, are we not falling away to the state of sheep? What do we destroy? The faculty of reason. When our actions are combative, mischievous, angry, and rude, do we not fall away and become wild beasts?"

If we recklessly indulge in pleasures, or fall prey to violent emotions such as rage, we forgo our humanity.  This doesn't mean that we should navigate life by suppressing our emotions and sporting a stiff upper lip. But it does mean that our pleasures should be moderate and well chosen, and that we should be in charge of them, not the other way around. It also means that we should strive to cultivate healthy emotions: not rage, but a sense of justice; not lust, but love; not fear, but cautious understanding.

It should be clear from the above that our prohairesis is inherently dynamic. If it were not, we couldn't change it. Moreover, this dynamism has a peculiar characteristic: it is recursive, meaning that we can apply our faculty of judgment to itself, and improve it over time, with practice and attention.

The Stoic notion of the self, then, is that it is a dynamic process, which modern science tells us is grounded in specific anatomical structures of the brain, like the prefrontal cortex.

Interestingly, modern neuroscience has confirmed the existence and importance of something along the lines of the Stoic prohairesis, though it turns out the Stoics were spectacularly wrong about its anatomical correlate. They thought it was made possible by the heart, but what modern scientists call the executive functions are in fact dependent primarily on the prefrontal cortex of our brains, though other areas, such as the caudate nucleus and the subthalamic nucleus also play a major role because they mediate inhibitory controls (you know, like the one I exercised when I rejected my initial impression of the gelato).

The Stoic notion of the self, then, is that it is a dynamic process, which modern science tells us is grounded in specific anatomical structures of the brain, like the prefrontal cortex. The self is not made of any essential and permanent component, and it obviously dies with our body. It is, however, the very subprocess within our body that truly makes us who we are. I can lose a leg or an eye and still very much be me. But if a neurodegenerative disorder significantly impairs or destroys my prohairesis, I'm done as a human being. Conversely, the best thing I can do for myself and others is to constantly work to improve my prohairesis, in the continued quest to become a better member of the human cosmopolis.