How important is this: A short description of scientific words like theory law and fact.

This is inspired by some recent arguments I've read between creationists and people with a solid understanding of science.

It became obvious that the arguments are, well, complicated by each side using different language. On the whole, the creationists either deliberately twist either the meaning of a word or extrapolate too much from part of its real meaning.

So I've written this as a point of reference.

This is the biggie. The word theory gets downgraded and misused especially in creationist language.

What they mean by: "It's only a theory" is that a theory is somehow inherently untrustworthy, and should not be relied upon. The implications are that a theory is somehow fragile and subject to sudden collapse on a whim at any time. That can be true of poor theories. It can be true of theories that are not based on fact. It is true of a belief that is presented as a theory, but it it not true of good, powerful well tested long-standing theories supported by thousands and millions of observations. However, the rules of logic dictate that even a well proven theory is constantly under scrutiny.

The real scientific meaning is as follows:

A theory is an explanation.

That's a pretty easy statement to understand. So what is the fuss? Well, before I continue, we need to understand some more words properly.

Statement.

A statement is a collection of words that we can evaluate in order to get TRUE or FALSE.

For example, these are all statements, all of which are FALSE

"A cat is a house."

"No frog can hop."

"Purple is a flute."

"You cannot place things on any table."

"The Earth was created in seven days."

Axiom

An axiom is a statement that is self-evidently true. It is considered so obvious that further statements, theorems and proofs thereof may be constructed and also taken to be true... which leads me to:

Logical argument

A logical argument is a way to combine statements to create new statements. An example is:

1. Dogs are mammals.
2. Most mammals give birth to live young but the rest lay eggs.
3. Dogs do not lay eggs.

Therefore, Dogs give birth to live young.

The tricky bit about logical argument is that the introduction of a false statement affects the truth of the conclusion. I had to be careful to QUALIFY the statement about mammals giving birth to live young. If I omitted the word 'Most', then it would be ambiguous and a useful statement cannot be ambiguous. If I had said, "ALL mammals give birth to live young." This would be incorrect".

Furthermore, a conclusion, even if true might not be a valid conclusion if the argument is not true for every step. It's possible for two or more false statements to cancel each other out.

Another related example is:

1. Dogs give birth to live young.
2. Every animal that gives birth to live young is a mammal.

(1.) is true.

(2.) is false and so we cannot conclude that a dog is a mammal based only on this line of reasoning.

In this case a third observation about the world invalidates (2.), namely:

3. Most sharks give birth to live young.

Logical arguments based on axioms are obviously subject to the ultimate truth of those axioms. This is why an axiom should be chosen as self-evident and obvious.

Fact

Facts are statements that are true. Statements that are true become fact when every experiment, every observation and all valid reasoning supports the truth-i-ness of the statement. Theories are built around facts, otherwise, they are not theories, they are beliefs.

If a statement is made and declared true, then it is declared universally true. There is no cop-out that can be swept away or ignored. Either a statement is true, or it is false. Just because it is declared true is not a guarantee of that status. You need to validate the truth of the statement (or prove it if possible).

If a theory works for most but not all observations, then scientists know that the theory is either wrong or incomplete, or there is an error in the exceptional observations. Even if a theory has such problems, providing you state certain reasonable assumptions, it is still worth using and developing until a better one is found.

Validation

Validation is the application of supporting evidence to a statement. Let's take the example:

"All mammals lay eggs."

This is supported by the presentation of the Platypus as an evidence. We know the statement is false, but if there is only limited evidence, then it may appear true. Under no circumstance would we declare this an axiom or a fact because it is not self-evident or obvious and there is not much supporting evidence. To support the truth of this statement, we need lots of evidence from diverse sources under many conditions over a long period of time. It can be hard to strongly validate the truth of a statement.

Falisfy

The statement "Mammals lay eggs" can be falsified by thousands of observations. However, we only need one to do the job. "Cows are mammals that give birth to live young". We can now use syllogism to conclude that "All mammals lay eggs." is FALSE, even though you could never have seen ALL mammals giving birth. The one cow that gave birth to live young is evidence enough to falsify the statement.

Law

As supporting evidence builds up, a statement might become incredibly well supported over a long period of time. If the truth of a statement is validated so well then it becomes possible to use it to invalidate other statements to a high degree of certainty. A law is also endowed with elegance and simplicity.

A law is almost axiomatic, but not self evident and must benefit from lashings of supporting evidence. The 'law of gravity' is one such law.

Realm of validity

A law underpins thousands of theories. But a law need not be absolute. It is a law because thousands of theories that have thousands of supporting observations of many years reply on it. If you find an exception to a law within its realm of validity, then ALL theories that rely on it must be thrown away. If someone invalidated the law of gravity, this would certainly crush tens of thousands of theories. It would be the biggest upset in all of science.

BUT as in the case of Newton's law of gravity which has a realm of validity for everyday observations, scales and non-relativistic speeds, within an extended realm, such as speeds near c, and near extremely massive objects, the law of gravity is cloaked by a new theory to explain those extreme cases. Einstein did not invalidate Newton's laws of gravity. He created a new theory with a more far reaching realm of validity that incorporates Newton's laws. It's incredibly important to understand this. Einstein did not prove Newton's laws wrong. He found a theory that works over a wider realm of validity.

If one single case found within Einstein's theory invalidates Newton's laws of gravity within its respective realm of validity, then it is not Newton's laws that would come crashing down, it would be Einstein's theories.

You can't easily argue with a law. It has been declared a law because of the inability to argue with it.

Any theory is an attempt at an explanation. There are probably millions of people every day clinging to their own private theories to explain what they experience. This might be a private theory about why their spouse squeezes the toothpaste at the top of the tube, or some whacky theory that life on Earth was seeded by aliens.

But a tested scientific theory is much more. A tested scientific theory is an explanation that makes sense of a class of observations. It is based on facts. It does not break laws. It can be presented as a logical argument where every step is supported by evidence or logical conclusion. A scientific theory is always under review. A scientific theory is our best explanation and is capable of acting as a model.

A theoretical model is successful if it is a good enough approximation to reality that it is possible to play with the model and make predictions about reality.

Evolution is an observation which has been observed thousands of times and may be considered fact.

A theory of evolution attempts to explain the natural world, and it does so spectacularly well. It does not violate the laws of nature, and it guides further investigation in a predictive manner to find out more about the world. No observations since the inception of Darwin's theory of evolution has come close to invalidating the theory.

As such, our understanding that is a theory of evolution is among the most reliable and well tested scientific theories of all time.

That is an amazing and exalting achievement for something that is 'only a theory'.