Only Harris is making an assertion.
Incorrect.
Harris is asserting that she had a job in college.
Trump is asserting that Harris is lying.
Trump doesn't need to provide evidence that a claim is false.
He is claiming Harris is lying. I have given a couple of ways he could evidence that. He repeatedly shows he doesn't care about evidencing claims, but that doesn't mean it's not possible for him to do so.
Harris needs to provide evidence that it is true.
That may not be possible. I have explained why. Again, her claim is not an outlandish one in the same way that "I walked on the moon" is.
Many kids get jobs in college, not many people walk on the moon.
What someone says, without evidence, is not true in and of itself.
Correct.
By default, if she cannot support her claim, then she is either lyineg or honestly mistaken.
Incorrect. I went on holiday to Majorca when I was 14. I don't believe I have any photos of that, I'm sure some were taken at the time but imagine they've long since been lost. So I cannot support that claim. That doesn't mean I'm lying or mistaken. I'd suggest that your experience of the world - that people do often go on holidays - and your knowledge of me - that I'm in the UK and therefore Majorca is relatively close to me and a common holiday destination for Brits - should be factors in how credible you find my claim.
Why do you think it's true? Just because she said it? I find that comical.
Well, no. Not just because she said it. As I said, if she said she'd walked on the moon then I'd not believe her until she supplied some good evidence. I believe her because she said it AND because I don't see why she'd lie about it AND because no-one who knows her has called her out on it being a lie AND because it's a plausible claim - again, kids having a menial job while at college is very common. So it's a combination of things.