Most people see being honest and lying as two extremes in black and white. But I think it is more accurate to conceptualize it as a spectrum with shades of grey. Being truthful is different from being correct or accurate in your statments, it is more about your beliefs when you make a claim.
People are said to have told a lie when they make an assertion that they believe to be false. So there are two conditions for something to be a lie. This is too narrow of a definition since white lies satisfy this definition but are ethical and lying by omission is not a "lie" but is immoral.
A lie is not always immoral, sometimes the intent behind the lie is positive. Someone might lie about a suprise party to maintain the surprise. Most pragmatic people understand since the purpose of the lie is not malicious, it is not unethical.
But who decides if the outcome is good for you? It is a slippery slope when we start justifying lies saying it is for his benefit. A girlfriend might lie about her sexual history because she thinks it is better for him to never know, but if he ever finds out he will feel betrayed. Was that a white lie? maybe a nobel lie? We need to be able to predict the counterfactuals accurately to be sure.
Let us examine the first condition, does it have to be a strong assertion? People like scientists find it hard to make strong claims, they use qualifiers and talk about how "there is a chance that", "it seems likely", "the evidence suggests", etc. But even when people leave themselves an out and make soft claims, if we find out they were saying something they did not believe, we still call it lying.
This is because they were misrepresenting their beliefs. The important condition is always if their words match their true internal beliefs. When people lie, they seek to manipulate, get the victim to operate based on incorrect information. They communicate in bad faith.
If someone themselves were told the wrong information and they believed it then they are not lying. They believed the falsehood to be true when they passed it on so the intentions are clean here. They were just used by the creator of the lie to spread falsehood. It could also be a misunderstanding. But it is hard to know internal states of humans, anyone could claim to have been deceived themselves.
Misleading people is a very useful tool in the arsenal of con-men, scamsters and tricksters. When you want to cheat or deceive people it works better when you set up circumstances leaving clues for them to find. Discovery always beats disclosure if you want them to trust the data. It is sort of like inception, because you want the mark think it was his idea. These criminals don't do the thinking for the victim, telling him what to believe is a very crude, blunt lie.
So lying fundamentally works by distorting the "map" used by the victim. If he believes us we change the reality of that person, we leverage his trust to change the data he bases his decision on. But it does not have to be new incorrect data, by holding back relevant data you can do the same damage
The legitimate way to change the behaviour of a person to communicate and change the algorithm he applies. You can also bring his attention to relevant true data, but what if you cherry pick the data? What if you decide to not inform of information that you know would be of use to him.
Lying by Omission
Many people try to claim they did not lie based on a technicality, "I did not say anything false, I just withheld relevant information.", "They did not ask me", these people phrase their answers in a way that is technically true but it is a lie in spirit since they knew what the person was asking but chose to give them incomplete data to work with.
Often they answer ambiguously in an evasive way trying to change the direction of the conversation. We all do this, try to shift the topic away from uncomfortable topics. This is not lying but it is not being honest either. This is where the shades of grey are apparent.
In fact the famous oath used by witnesses explicitly states "the whole truth", which means withholding information is pejury.
“Do you swear to tell the truth, the whole truth, and nothing but the truth?”
But in practice is is hard to prosecute lying by omission. When people think in black and white, using rigid categories often some grey items like these are misclassified.
Lying by omission is one such grey area which should stay in the black region and not go to the white area.
There are even more levels to this. The only way you can maliciously hold back the whole truth is if you had access to all the relevant facts.
Is it a form of lying to consciously avoid learning the relevant facts? You might do this when you decide in advance what you want and just search out supporting evidence. So you can honestly answer all the questions because you really don't know anything more.
Say you want to have kids, you don't look into all the risks, the potential issues. So when you discuss with your wife you can honestly convince her to go ahead. The onus is on her to find out the potential risks.
This is why it is important that the both of you share the same goals in the same priority - health and happiness of the mother over having kids. Then you will surely look for all the relevant information that she will want to make an informed decision.
This does not even have to be conscious, we subconsciously avoid uncomfortable truths. We like echochambers and don't enjoy challenging our preconcieved notions. Confirmation bias affects all of us, even influencing what we remember!
The purpose of lying is not always malicious, it could be to maintain privacy. My fetishes, my taste in women, income, weight, sexual history, etc there are many things which people agree I have a right to hide. To effectively achieve this I need to deploy some level of deception. I can't answer "I don't want to talk about it" in a blunt way if someone jokes about me liking to fuck animals. That would be a dead giveaway. So context is very important when we judge the morality of lying.
I see the spectrum of lying going from ethical to unethical as,
lies to protect privacy,
redirecting conversation topics,
learning in a biased way to reinforce beliefs,
lying by omission,
Ideally we should all strive to be as honest as possible. Make sure our words and actions represent our true beliefs.