“Who am I?” If Google’s autocomplete is any indication, it’s not one of the questions we commonly ask online (unlike other existential questions like “What is the meaning of life?” or “What is a human?”). But philosophers have long held that “Who am I?” is in some way the central question of human life. “Know yourself” was the inscription that the ancient Greeks inscribed over the threshold to the Delphic temple of Apollo, the god of wisdom. In fact, self-knowledge is the gateway to wisdom, as Socrates quipped: “The wise person is the one who knows what he doesn’t know.”

The reality is, we all lack self-knowledge to some degree, and the pursuit of self-knowledge is a lifelong quest—often a painful one. For instance, a common phenomenon studied in psychology is the “loss of a sense of self” that occurs when a familiar way of thinking about oneself (for example, as “a healthy person,” “someone who earns a good wage,” “a parent”) is suddenly stripped away by a major life change or tragedy. Forced to face oneself for the first time without these protective labels, one can feel as though the ground has been suddenly cut out from under one’s feet: Who am I, really?

But the reality of self-ignorance is something of a philosophical puzzle. Why do we need to work at gaining knowledge about ourselves? In other cases, ignorance results from a lack of experience. No surprise that I confuse kangaroos with wallabies: I’ve never seen either in real life. Of course I don’t know what number you’re thinking about: I can’t see inside your mind. But what excuse do I have for being ignorant of anything having to do with myself? I already am myself! I, and I alone, can experience my own mind from the inside. This insider knowledge makes me—as communications specialists are constantly reminding us—the unchallenged authority on “what I feel” or “what I think.” So why is it a lifelong project for me to gain insight into my own thoughts, habits, impulses, reasons for acting, or the nature of the mind itself?

This is called the “problem of self-opacity,” and we’re not the only ones to puzzle over it: It was also of great interest to the medieval thinker Thomas Aquinas (1225-1274), whose theory of self-knowledge is documented in my new book Aquinas on Human Self-Knowledge. It’s a common scholarly myth that early modern philosophers (starting with Descartes) invented the idea of the human being as a “self” or “subject.” My book tries to dispel that myth, showing that like philosophers and neuroscientists today, medieval thinkers were just as curious about why the mind is so intimately familiar, and yet so inaccessible, to itself. (In fact, long before Freud, medieval Latin and Islamic thinkers were speculating about a subconscious, inaccessible realm in the mind.) The more we study the medieval period, the clearer it becomes that inquiry into the self does not start with Descartes’ “I think, therefore I am.” Rather, Descartes was taking sides in a debate about self-knowledge that had already begun in the thirteenth century and earlier.

For Aquinas, we don’t encounter ourselves as isolated minds or selves, but rather always as agents interacting with our environment.

Aquinas begins his theory of self-knowledge from the claim that all our self-knowledge is dependent on our experience of the world around us. He rejects a view that was popular at the time, i.e., that the mind is “always on,” never sleeping, subconsciously self-aware in the background. Instead, Aquinas argues, our awareness of ourselves is triggered and shaped by our experiences of objects in our environment. He pictures the mind as as a sort of undetermined mental “putty” that takes shape when it is activated in knowing something. By itself, the mind is dark and formless; but in the moment of acting, it is “lit up” to itself from the inside and sees itself engaged in that act. In other words, when I long for a cup of mid-afternoon coffee, I’m not just aware of the coffee, but of myself as the one wanting it. So for Aquinas, we don’t encounter ourselves as isolated minds or selves, but rather always as agents interacting with our environment. That’s why the labels we apply to ourselves—“a gardener,” “a patient person,” or “a coffee-lover”—are always taken from what we do or feel or think toward other things.

But if we “see” ourselves from the inside at the moment of acting, what about the “problem of self-opacity” mentioned above? Instead of lacking self-knowledge, shouldn’t we be able to “see” everything about ourselves clearly? Aquinas’s answer is that just because we experience something doesn’t mean we instantly understand everything about it—or to use his terminology: experiencing that something exists doesn’t tell us what it is. (By comparison: If someday I encounter a wallaby, that won’t make me an expert about wallabies.) Learning about a thing’s nature requires a long process of gathering evidence and drawing conclusions, and even then we may never fully understand it. The same applies to the mind. I am absolutely certain, with an insider’s perspective that no one else can have, of the reality of my experience of wanting another cup of coffee. But the significance of those experiences—what they are, what they tell me about myself and the nature of the mind—requires further experience and reasoning. Am I hooked on caffeine? What is a “desire” and why do we have desires? These questions can only be answered by reasoning about the evidence taken from many experiences.

Aquinas, then, would surely approve that we’re not drawn to search online for answers to the question, “Who am I?” That question can only be answered “from the inside” by me, the one asking the question. At the same time, answering this question isn’t a matter of withdrawing from the world and turning in on ourselves. It’s a matter of becoming more aware of ourselves at the moment of engaging with reality, and drawing conclusions about what our activities towards other things “say” about us. There’s Aquinas’s “prescription” for a deeper sense of self.