Imagine laying in bed at night and a cringe memory from two years ago flashes in front of your eyes, making your heart race and palms sweat. Or an argument from your past makes you think of rebuttals you could have clapped back, leaving you lamenting after better articulation abilities. Mental notes, reminders, even full-blown conversations happen in people’s minds all the time, in images, in words, in elaborate scenarios, as they go about their daily routines minding their own business. It’s called having an internal monologue, and it can say a lot about people.

Internal dialogue, inner voice, or private speech often develops in childhood, as children learn languages and figure out how to make decisions independently. The most common way it manifests in childhood is in the form of imaginary friends. Verbal chatter is the most common manifestation of inner monologue that scientists have been able to document. It kicks in when you’re waiting your turn to speak out loud, or when you’re silently disagreeing with someone, or when you’re making a mental checklist of what to buy at the grocery store. It can happen when you’re singing a song to yourself, or thinking of a movie quote, or reading silently as you go over the words in your brain.

This, however, doesn’t happen to everyone, or at least not in the same way. In January 2020, a blog post kicked off a viral debate about internal monologues by stating some people simply don’t have an inner voice. This brought to the forefront experiences of several people who didn’t engage in self-talk at all. But did that mean they didn’t have an internal monologue?

Scientists have found an internal monologue can be more than just talking to yourself in your head. It can be an integral part of how people perceive their surroundings, their own selves, their thoughts, and memories. And so, it can manifest in different ways in the brain that are not necessarily verbal, or so clear-cut and easily identifiable. Psychologist Russell Hurlburt has studied internal monologues at length and found “most people think that they think in words, but many people are mistaken about that,” he tells BBC Future. Hurlburt found people, when asked what was going through their minds at any particular moment, couldn’t always accurately describe it, a phenomenon he explains by the fact that we don’t necessarily stop and consider our inner monologue most of the time, on a day-to-day basis. It’s one of those things that just happens in the background, demanding no attention or care from our conscious brains.

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To explain the varieties of the inner monologue he has documented over the years, Hurlburt devised five categories — inner speech, which is what most people think is the extent of the internal monologue; inner seeing, which constitutes images (both real and imaginary) that helps people think and remember; sensory awareness, which helps people understand their surroundings through the registering of touch and feel; feelings, such as happiness and sorrow; and finally, unsymbolized thinking, which is a thought that registers, but doesn’t necessarily manifest as any of the four above. Taken together, an internal monologue is the way in which we identify and make sense of our thoughts, the process for which can involve all the ways in which we’ve learned to use our senses.

“Internal monologue probably comes in degrees of abstractness,” linguistics expert Mark Scott tells Today. He found this process to be facilitated by a brain signal called ‘corollary discharge’ that helps to differentiate between different sensations and experiences, and whether they’re coming from the external environment, or your internal thought process.

And the internal monologue can serve a variety of different purposes — it can be that self-critical voice in your head supplementing your anxiety, or it can be one that helps you prepare yourself for an ordeal or one that harbors different versions of your personality that you can invoke for an impromptu pep talk at any given moment. At the end of the day, hearing voices in your head isn’t all that uncommon, and turns out it’s an integral part of how we understand our own thoughts.