What is an Idea?
An idea is the smallest unit of coherent thought. However, the word "idea" often refers to multiple ideas. We can thus ask questions about how many ideas are an idea. It sounds weird, but it's a pretty good question. Suppose I have an idea about how to write a new computer program. It's completely legitimate to call that one idea in English. But it can also be divided into parts, such as different steps I will perform to make the program. Each of those parts is also called an idea and consists of various sub-tasks which are themselves complex ideas consisting of yet smaller ideas.
No one knows a good method of counting ideas, or deciding where one ends and the next begins. Until someone figures that out, we'll have to be a little imprecise about it.
A "theory" always refers to multiple ideas, not that that tells you much. This is an explanation of what a "theory" is by David Deutsch. It equally well explains ideas.
Say you asked Buffy fans which is better, the movie or the TV series. Most of them would say the series. And if you asked them why, they would give a lot of the same reasons.
So, these people are all alike in a certain way. Some of them are physicists, some are axe murderers, some are men, some women, some neither… And so, only part of them is alike and the rest of them is different.
Think about the part of them that is alike. An interesting fact about this is that it behaves the same way, even though it is located in brains which, in other respects, hold very different contents. And that tells us that this part of the brain — the part of a Buffy fan's brain that judges whether the film is better than the series — does not consult the rest of the brain when making such decisions. In other words, it operates autonomously.
It's not completely autonomous. The more detailed the questions you ask about why the movie is worse, the more different the responses will be. But approximately speaking, it is autonomous.
And that nearly-autonomous thing is not a person. It is much less than a person. I call it a theory. This is a misleading term in many ways because when people think of 'theory' they usually think of something like Newton's theory of gravity.