I was asked recently whether I thought it was possible to completely trust someone in a relationship not to willfully harm you. But of course, we have to know what we mean by "trust" if we're going to answer that question.
It seems to me that "trust" is a question of knowledge. Can we know that someone we're in a relationship with, a partner, will avoid causing us harm? Some may claim that unless we can know with absolute certainty that a partner (or friend, or family member, etc) will not cause us harm, then we cannot trust them. That's a tall order. For there to be such a guarantee, a person would need to have perfect knowledge-- to be omniscient. This is not possible, even conceptually.
Any being that would claim to be omniscient could be mistaken. There could be something that the being didn't know, and didn't realize it didn't know, or even realize that there was something to know at all. As such, this being would think it was omniscient, without being omniscient. It would be mistaken.
So are we doomed to never trusting anyone, simply because there is no perfect knowledge? I don't think so. There are many things that we can claim to know, even though we might be wrong. I know that I'm sitting in front of my computer, on a chair, typing these words. It's possible that I'm simply dreaming, or hallucinating, or trapped in the Matrix, but while the possibility of any of these is non-zero, that doesn't mean that I must claim that I do not have knowledge that I'm sitting in a chair typing these words.
Knowledge simply doesn't need to be at a level of 100% to be considered knowledge. If it did, we could not ever claim knowledge of anything, not even scientific claims that have mountains of evidence. For example, I know that the speed of light is the maximum speed of anything in the universe. Every piece of evidence acquired to date about the properties of light, matter, space, and time, point to the speed of light as the maximum speed attainable in the universe. But even that is not 100% certain. Not that long ago a group of scientists thought they had found faster-than-light neutrinos. Had that panned out, decades of physics research might have been brought into question as scientists tried to square all the evidence of the past with this new evidence (it turned out to be an equipment error, by the way). If we find ourselves in a situation where our knowledge is only 99.99999999% certain (such as the speed of light), we don't have to declare that a mere belief, or that we don't really know the speed of light. We do know. We must be cognizant of the fact that our knowledge could be proven wrong if new evidence comes to light (and is confirmed), but we can still say that we know with a high confidence level, such a high confidence level that it's as close to 100% as we can get. (Hat tip to Dan Fincke for the argument that I borrowed from for this part)
Trust is like that. Sure, we can never know with 100% certainty that a particular person in a relationship with us won't hurt us, or even harm us at some point. We can't even know with 100% certainty that they won't willfully hurt us. But we can take all the evidence of character that we've ever obtained or witnessed about that person, and rationally assess that they are trustworthy, and will not willfully hurt us, and especially won't harm us.
Let me give an example. I've known my wife for roughly eight years. In all that time, I've never been apprised of a situation, or seen evidence of a situation, that would suggest that she isn't worthy of trust. What I have seen is her doing things like asking if she can share something private with others before she does, and, if I say no, I've seen her refraining from sharing it. I've seen her not take opportunities to dig at me in hurtful ways, even when she was angry or upset, even though she has tons of ammunition at her disposal. I've seen her tell me whenever she had a crush on someone, and she's never hesitated to tell me how those crushes are developing when asked. I can say without hesitation that I trust her completely.
Yes, there's technically an outside chance that I could discover she's betrayed my trust. But frankly, I think I'd have a better chance at winning the lottery jackpot. We can, and ideally should, make the effort to apportion our beliefs to the available evidence, but when that evidence accumulates, at some point we can say that we know, and in relationships, that we trust.