Re: What is the rigorous scientific definition of 'In Principle' ?

Hi Ray,

Thank you for your question. I'm going to attempt to answer it, but this is I think my personal take on the subject, and not necessarily the opinion of other scientists. This is sort of an answer already: while there are "rules" language in papers should abide, these rules are not as hard as the rules of physics themselves. I'll qualify this statement a bit, and then explain some examples in which we might use "in principle".

There are certain "rules" to writing scientific papers. In principle, papers should be free of opinion and based in fact. This is also reflected in the choice of words. For instance, I might be writing a paper on a plasma that is no larger than a needle. Now, I might think this is a pretty tiny plasma, and, in fact, I think it is, especially compared to most plasmas. However, someone investigating an even smaller plasma will disagree with this, and may misunderstand parts of the paper.

This brings me to the first use of the term "in principle". It can be used to indicate that something is often true, but there are exceptions. For instance, I could comment that a certain factor is small. Presumably, I mean that it is small compared to other factors, but whether or not, say, 10% of other factors is small or not depends on the situation and the reader might disagree with me. Whether this is an opinion or a fact is a bit ambiguous; I'd be lying if I called it "large," it is certainly smaller than other terms, but is it actually "small"? Words like "tiny," "huge" and "gargantuan" are right out unless they are names, because it is really hard to tell what "tiny" means. 10% or less? 2% or less? 0.1% or less? I hope this serves to illustrate the use of use of objective language in papers. It's a bit of an art, and it takes a while to get it.

Let's look at your example next. When a cup breaks, the surface reorganizes itself a bit. It also oxidizes, most likely, and may react with water in the air as well. This might passivate the surface. I suspect that you can't re-assemble a cup like this in practice. In principle, however, if we were to perform this experiment with a piece of metal, in a vacuum, and press really hard, we might have more success. So in your case, there is a general scientific principle, which says that we can reassemble the cup, whereas there are some practical issues that preclude this.

I think this is the most common and best use of the term "in principle", namely, to oppose it with "in practice". In principle, I should use general relativity to compute how long it will take me to fly from Amsterdam to Chicago. In practice, the difference is so tiny that it is for all practical purposes negligible. So, I use Newtonian physics. In principle, pure water is a poor conductor of electricity. In practice, water is rarely pure, so using electrical appliances while sitting in a bath is not a good idea. I think you get the gist. It might also mean that it will take impossibly long for something to happen. In principle, the coffee in my cup might jump up, spell the Schroedinger Equation and then fall back into my cup. The odds of it happening are small beyond human comprehension.

In short, the "in principle" means that something may happen according to a certain theory, but that in practice, something else happens for some reason. I hope this answers your question.

Regards,

Bart Broks