Oddly enough, one of the most important aspects of science is also one of those most frequently misunderstood. That, of course, is the concept of a "theory."
The problem is that this is one of those words which has two meanings. There's the common meaning, which is much like what a scientist would call a "hypothesis." Then there's the scientific meaning, which is much, much more. In order to get to the difference, we need to look a little bit at just what it means to "know" something.
We learn our universe by experience; pretty much all of our useful knowledge is experiential. As we bump around, encountering things in our world, our brains collect enormous numbers of bits of information--"facts," we call these. They're just little, repeatable observations.
As our library of fact information increases, our brains do something which is apparently deeply innate to us: they form patterns. The patterns that our minds create out of those bits of information are what we "know" about the universe. Usually these patterns are pretty good reflections of reality; sometimes they can be spectacularly wrong.
Thus, if I asked you what you knew about ice, you'd have no hesitation telling me that ice is cold. How do you know this? You've had a lot of experience with ice in your lifetime. And every piece of ice you've ever touched has been cold. So your brain draws the conclusion--whether it's merited or not--that all ice is cold.
In fact, under the right conditions, water will freeze at room temperature, so while your knowledge about ice is very useful to you, it isn't actually completely correct.
This bit of knowledge--that ice is cold--is what we call a posit. A posit is a generalization drawn from observations. Note that, like the ice example above, it is impossible to know for sure that any posit is absolutely correct. However, it's something for which we have so much evidence that it is safe and useful to treat it as if it is correct. You are not liable to get into any difficulties in your life if you assume that every bit of ice you encounter is cold.
Virtually all of the information we have about our universe consists of posits. The ones we gather by our normal bumbling about we lump together and call "common sense." But scientists go about gathering their posits in a much more stringent and organized fashion than this. And they are often seeking experience in areas that we don't normally encounter in the every day world, using highly specialized ways to analyze what they encounter. This is why so much scientific knowledge is so counter-intuitive (meaning that it doesn't match our common sense expectations of the universe). We don't normally have all the evidence.
Here's an example. If you've got anything in your hands, put it down for a moment. Now use one hand to grasp and feel the other. Does it make any sense to you that your hand--that solid, substantial object--is actually made overwhelmingly of empty space? And yet, every method physicists and chemists have devised for looking at matter says that it is. Surprise ;^)
So our knowledge of the universe is experiential--a huge collection of posits we've made based on the things we've encountered in our lives. And scientists are in the business of formulating much more systematic posits, based on kinds of experience we normally don't encounter.
Scientific knowledge is composed of two kinds of posits: laws and theories. Yes, scientific laws are posits, which means that we are not certain that they are true.
So what's the difference between a "law" kind of posit and a "theory" kind of posit? Contrary to the words of many, good theories don't grow up to be laws. They are quite different in nature and purpose.
Scientific laws are descriptions of the ways that matter and energy behave. Laws are often expressed best mathematically, since they represent observed patterns of behavior. For example, in the early days of modern chemisty, scientists observed the ways in which different elements combined with each other. They had no notions about how atoms interacted--in fact, they didn't even have the notion of atoms yet. But they were able to observe some very consistent behaviors. For instance, hydrogen and nitrogen will combine together to form ammonia. But there's nothing particularly random about it--it always uses up three times as much hydrogen as nitrogen. Three parts H to one part N. And when hydrogen combines with oxygen to form water, it always takes twice as much hydrogen as oxygen. Two parts H to one part O. And carbon and hydrogen combine in the ratio of four parts H to one part C. Oxygen combines with carbon in the ratio of 2 parts O to one part C. Over the years, many, many of these relationships were discovered, recorded and shared. These became the Laws of Chemical Proportion. None of these budding chemists had any idea why these elements combined in these proportions; their law simply described how the elements behaved.
It wasn't until this century that the reason for this behavior began to emerge--with the formation of our modern Theory of Atomic Structure. What we now believe we understand about the way atoms are built--a theory--accounts for all of those ratios of combination--a law.
And that's what a theory is--an explanation. Laws describe; theories explain. And it's the explanatory power of a theory that makes it powerful and important.
Ideas which a scientist honors with the title of theory share a number of features:
Theories are very important ideas--arguably more important than laws. Laws are rather like tools; theories are the working concepts of science.
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