The term comes from the 1944 film Gaslight, in which a husband dims the gas lighting in the house and then insists his wife is imagining it, slowly convincing her she's losing her mind. Clinically, gaslighting describes a pattern — not a single lie — of repeatedly denying someone's documented experience until they stop trusting their own perception and start outsourcing reality checks to the manipulator. It requires two ingredients working together: consistent denial of things the other person clearly experienced, and enough isolation from outside perspective that there's no one else to confirm what actually happened.
The reason gaslighting works even on intelligent, otherwise-perceptive people is that it doesn't announce itself as an attack — each individual denial is small enough to seem like a misunderstanding ('that's not what I said,' 'you're remembering it wrong'), and it's the accumulation over months that erodes confidence. The most effective countermeasure isn't arguing harder in the moment — it's keeping an independent record (a journal, texts, a trusted outside friend) that exists outside the manipulator's narrative, so there's a fixed reference point that can't be quietly edited after the fact.
Among the tactics Simon names from his clinical practice, denial is one of the most corrosive: the covert-aggressive person flatly refuses to admit something happened, even when there's little real doubt that it did, forcing the other person to keep re-litigating settled facts. He pairs this with minimization — "you're blowing this way out of proportion" — so that even a rare partial admission gets reframed as the victim's overreaction rather than the manipulator's behavior. Simon's patients arrived at his office years into this pattern, genuinely uncertain whether their own memory could be trusted, not because they were fragile or unstable, but because the pattern is specifically engineered to produce that exact doubt over time.