Why it's called "borderline"
If you've been close to someone with BPD, you probably know the moments I mean. They were certain you were cheating. Certain you said something you never said. Certain an event happened a way it demonstrably didn't. Not suspicious. Absolutely certain. Even within a single conversation, reality would shift, as if what had happened twenty minutes earlier no longer existed. No amount of evidence, calm explanation, or receipts could touch it. I walked away from those conversations questioning my own memory because of how certain they were in their version of things, and because of how much I trusted them.
Sharp explained where the word "borderline" actually comes from. It's not describing a person who is borderline-anything in the casual sense. It's a clinical border, the term originated to describe patients who sat between neurosis and psychosis.
The history is worth a minute. For most of early psychiatry, patients went into one of two bins. Neurotic meant anxious, depressed, conflicted, but in contact with reality. Psychotic meant a break with reality. In 1938, a psychoanalyst named Adolph Stern described a group who fit neither bin. In the office they looked neurotic, but under stress (often the stress of the therapy itself) they would slide toward paranoid, reality-distorted states, and then come back. They lived on the border line, so that's what he called them.
The concept got sharpened over the next forty years. One analyst famously described these patients as "stably unstable" — the instability was the consistent feature. Otto Kernberg formalized it in 1967 as a level of personality organization sitting between neurotic and psychotic structure, researchers led by John Gunderson turned it into measurable criteria in the 1970s, and the DSM-III made it official in 1980.
By then the name was a fossil. It describes a 1930s sorting problem rather than the disorder's actual content, which is why other diagnostic systems have preferred names like "emotionally unstable personality disorder." The fossil nonetheless preserves a real observation. Under enough emotional load, these patients drift toward the psychotic side of the border.
That drift is what Sharp is describing. Her words: a person in full psychosis is certain they're seeing an image that doesn't exist. That's the most extreme form of what researchers call psychic equivalence (when whatever is in your mind feels identical to reality, with no gap between "I feel it" and "it's true.")
People with BPD typically don't have that in a full-blown, fixed way. But Sharp is direct that in severe presentations, under enough emotional intensity, they can edge toward that place. This is what I encountered.
Two things this understanding has helped me with:
- Trust in my memory. The gaslit feeling — where you doubt what you know happened — makes a different kind of sense when you realize the other person was reporting their internal state with the full confidence of their perception being accurate.
- It drew a line. Understanding the mechanism is not the same as accepting what it produced. The certainty had a cause, the damage was still damage. Both are true, and you don't have to pick one. And there's variance. Some people with BPD are more intentional in the harm they do, the same way cruelty and altruism vary in the general population. The mechanism explains a pattern. It doesn't adjudicate every act.
Eighty years ago, clinicians watched people cross back and forth over a border between shared reality and private certainty. I didn't know the term's history, but I spent too much time staring at someone walking that border without understanding what was happening.