This page contains at least two separate streams of conversation.
Wittgensteins Ladder - simplification (lie) as a means to achieve understanding. See also True Enough
Almost True - deliberate deception for other reasons, and whether it can be justified
The truth is messy and often very hard to deal with. So don't. Instead, tell a lie that emphasizes the important bits and disregards messy details. Metaphors are lies. All Abstractions Lie. Software Lies. Everyone lies.
www.idehist.uu.se
[Edit Hint: broken link?]
The trick is to make certain that your lie is useful. Make sure it gets you closer to your goal. And, of course, never forget that you're dealing with a lie.
I don't like the word lie in this context. lie implies deliberate deception. I don't think most people use abstractions to deceive.
The point of this phrasing is to make people understand that not telling the truth is not only acceptable in some circumstances, but downright important. The word is being used as a rhetorical device to provoke reaction and, one hopes, thought. One of the most honorable professions is Liar to Children, also known as Teacher.
It is lying - you know that the reality is different from what you are stating. That's the defining characteristic of a lie - you're stating something you know is untrue. High school physics teachers know (I hope) that gravity isn't Newtonian, but they will teach their students F=Gm_1m_2/d^2. "Lie" doesn't make the distinction between "barefaced" lies, "little white" lies, or lies told you "for your own good". Hence the need for such adjectives.
'A wary teacher will make sure to define science at the beginning of his class - that is, a process used to approximate the truth. Other lines of thought can think of themselves as the end-all and be-all, but a person using science must be prepared to throw away everything he has learned in the face of a more plausible truth.'
Teachers often don't know that much about the subject they teach (in the scope of things); This is not a bad thing necessarily, their expertise should lie more in the presentation of material than the material itself. Is it lying to present a simplification of your understanding of a simplifications of something? I think this attempted distinction is more obfuscation. The 'truth' is a slippery customer.
The truly useful lies are the ones that inch you closer to the truth. -- Walden Mathews
I do not mind telling the truth about lying; the ancient siren song of all lying is the sales pitch of usefulness. Nor do I mind an economic view of truth, what is rare is valuable. But, What bothers me are attempts to define lying from a truth-less point of view; if all is a lie, how can a Useful Lie be accurate? -- Don Turnblade
I thought a Useful Lie is what Captain Kirk and Mudd told Norman The Android to overload his circuits. -- Michael Leach
"Everything I tell you is a lie." (This was actually spoken by Spock, making apposite use of the Cretan Paradox.)
The truly useful lies are the ones that inch you closer to the truth.
I think that this sets a dangerous precedent. Often, you don't want to get into all the details. When I run into this, and the details aren't important at the time, I say the simplified version (the "lie"), but put disclaimers on it to show that it isn't gospel.
For example, consider Newton's laws of motion versus Einstein's laws. Einstein's laws are accurate, as far as we can tell (and not considering the quantum level). But it has all these v-squared over c-squared pieces in it.
The whole point of V^2/C^2 is that these factors become huge near the speed of light, and negligible at more mundane speeds. If you assume V^2/C^2 to be zero, Einstein's equations collapse to Newton's equations. So I feel proper presenting Newton's laws of motion as correct for most practical purposes.
I'm not lying. I'm simplifying, but I am showing that I am simplifying and implying that more complex versions, which you probably don't need, exist.
As you say, it's not lying; it's simplifying. Presumably you are simplifying to avoid obscuring some other truth with the extraneous details of this one.
There are those who claim that you are deliberately not telling the truth, and therefore you are lying. You choose not to call it lying because you believe you are justified and that it's a Good Thing. Most likely your avoidance of the term "lie" is based on your perception that lies are bad, what you are doing is good, therefore it can't be a lie.
Most people on this page are arguing the same point: not telling the entire truth is sometimes the best thing to do.
Truth is contextual. As long as you (1) define your context, and as long as (2) what you declare to be true does, in fact, work within that context, there is no lie. And even if those two conditions are not met, you aren't necessarily lying. You could just be ignorant or naive (which is the case 90% of the time, IMO). -- Svad Histhana
"Intentionally allowing someone else, who has a right to know the whole truth, believe (through action or inaction) something else then the whole truth." (from en.wiktionary.org 
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Thus it would seem... that if someone has not the right to know, it would not be lying. -- Anonymous Donor
- but a person using science must be prepared to throw away everything he has learned in the face of a more plausible truth... Yes... I think the one thing all here are missing, is that to claim something, an idea, is absolutely true, at any given level of complexity in its description, is entirely arrogant and presumptuous. By that definition, no truth can ever be illustrated as it is simplified (a lie) - and will always be simplified as we will never know EVERYTHING about IT. -- Brian.Baley
A critical problem with e.g. teachers is that they often fail to state (or do not realize) that they make simplified claims. The result is students who move on in the incorrect belief that what they know is the truth. (Remember that most people, in particular children, lack the critical thinking necessary to see through Useful Lies on their own.)
My advice: Always state explicitly when you use a Useful Lie. -- MichaelE
It would seem to me that a lot of things called "Useful Lies" on this page -- abstractions, in particular -- are actually models, and it's more useful to say All Models Are Wrong, but some are useful. This is the first -- and almost only -- things I learned in an Economics class I tried taking in a Summer term years ago, before it got canceled due to too-small of a class size, and I had to take the class in the Fall anyway.
Newtonian Physics, for example, is a wrong model of reality, but it's useful for calculating the orbits of big bodies, and for designing roller-coasters. Einstein's Relativity is also wrong, but it helps us where Newtonian Physics cannot: it keeps satellites in the sky, and tells us how Mercury orbits around the Sun. Quantuum Mechanics is also wrong, but it helps us to take into account electron tunneling when we design tiny electronic circuits. -- Alpheus
In recent seasons, the Doctor of Doctor Who has been rather good at these. For instance:
Doctor:
Imagine a great big soap bubble with one of those tiny little bubbles on the outside.
Rory:
Okay.
Doctor:
Well, it's nothing like that.
See Also: Just Make It Right, Wittgensteins Ladder, Everything Is Relative, Life Isa Big Messy Graph, All Models Are Wrong Some Models Are Useful, The Map Is Not The Territory
See original on c2.com