In her testimony on Thursday before the Senate Judiciary Committee, Christine Blasey Ford was highly credible. That’s not because she was certain of everything she said. It’s because she wasn’t.

Ford resolved many doubts about her sexual assault allegations against Brett Kavanaugh. Why didn’t she tell her story until 2012? Because that’s when, during a renovation, she had to explain to her husband why she wanted a second front door on their house. (As a result of the attack, she said she felt unsafe with only one exit.) Why had she talked only to congressional Democrats? Because her congresswoman was a Democrat. Why didn’t Ford’s friend, Leland Keyser, recall being at the house party where the assault allegedly happened? Because Keyser wasn’t in the room where it happened, so to her, the party was unmemorable.

Ford’s strongest asset was her trustworthiness. She earned it by not overselling her case. She was frank with the committee about fatigue, the passage of time, and other factors that impaired her recollection. And she was honest about what she didn’t know. Ford said she couldn’t be sure which of the two boys—Kavanaugh or his friend Mark Judge—had initially pushed her into the bedroom where the assault happened. That’s because the push came from behind. She said she didn’t know who had paid for her polygraph or how that cost would ultimately be covered. When she was asked whether anyone had spoken to congressional staffers on her behalf, she said it was possible. If she didn’t know something, she said she didn’t know.

She also used external evidence, when it was available, to re-examine her recollections and clarify what was true. At several points, she checked written records or consulted her attorneys to sort out dates or details. Rather than guess, she offered to look things up. Where her memory was unclear, she pointed to checkable information—Judge’s employment dates at Safeway, her location in a Walgreens parking lot during phone calls—to nail down where and when things happened.

Unlike Kavanaugh, who has repeatedly mischaracterized the statements of alleged witnesses—Ford’s story, he falsely told the committee, was “refuted by the very people she says were there, including by a longtime friend of hers”—Ford examined her own answers as she spoke and corrected them when she saw errors. At one point, after saying she hadn’t spoken recently to people involved in the case, she corrected herself, noting that she had spoken to Keyser. At another point, after describing a trip in which she had come “here,” she stepped in to clarify that she meant Delaware, not Washington.

Ford was frank—far more frank than Democrat senators have been—about political considerations in her decision to come forward. She stated bluntly that Kavanaugh’s emergence as a likely Supreme Court nominee had driven her to act. She wasn’t saying that she would make up such a story. She was saying that the cost to the country of having a predator on the Supreme Court had come to outweigh her fear of going public with a painful story she knew to be true. She even indicated that she had calculated, as she followed news of the nomination process, whether telling her story would be sufficiently likely to affect Kavanaugh’s confirmation—and therefore worth the personal cost of speaking out. You can hold these political calculations against her. But her candor about them speaks well for her candor in general.

In several exchanges during her testimony, Ford signaled that she was shifting from one level of confidence to another. She said she wasn’t sure about the arrangement of her polygraph or the timing of her grandmother’s funeral, but in the next breath she expressed absolute certainty that she had written every word of the statement used in the polygraph. She stipulated that her recollection of the general location of the house party was just an estimate, but in response to the next question, she answered firmly that she had consumed no alcohol before the party.

Given the opportunity to endorse other allegations or insinuations against Kavanaugh, or to say something bad about him beyond the boundaries of what she had reported, she refused. When she was asked whether she knew of him committing sexual assaults at other parties, she replied with a firm no. And she gave the same answer when she was asked whether he had ever engaged in improper sexual behavior with her on any of the other occasions when she had been near him.

All of which makes it striking that on the central questions, Ford was definitive. She remembered Kavanaugh and Judge being quite drunk. She remembered them going downstairs after the attack, bouncing off the walls and laughing. She was clear about which things she could only infer—the conversation downstairs while she was upstairs, for example—and which things she had directly observed and would never forget. One of those things was the bed on the right side of the room. Another was her assailant’s identity. Sen. Richard Durbin asked Ford: “With what degree of certainty do you believe Brett Kavanaugh assaulted you?” Ford replied: “100 percent.”*

I can’t say with 100 percent certainty that Kavanaugh assaulted Ford. Nobody who wasn’t in the room can be that certain. But when Ford says she’s 100 percent certain about something, I take it very seriously. And that’s because she so rarely says it.

Correction, Sept. 30, 2018: This article originally misidentified the senator asking the question about Ford’s degree of certainty in identifying Kavanaugh as her assailant. It was Sen. Richard Durbin, not Sen. Dianne Feinstein.