Can you patent an idea? Nope. That's the short answer.
Here's the longer one...
Over time, accepted word use and meaning have evolved to make crafting and interpreting patents consistently possible. This is true for any field of endeavor. For example, in basketball, a pick occurs when one player blocks another player with their body. The term "pick" would not have the same meaning between two miners. Nor would the term "pick' have the same meaning between two dentists.
So it is with the terms "idea" and "invention" in patenting. What most people mean when they ask me whether they can patent their idea is whether they can patent their invention. However, I can't take for granted that what the inventor means and what I understand are the same. So, it's important to clarify what the inventor means when they use the term, "idea."
At this point, when explaining this sort of thing to a normal person, they get a far away look in their eye as if contemplating a piano falling on my head.
In patent law, an idea is a concept as in, I have an idea to make lawn mowing easier. An invention is the idea developed into something that actually does something as in an automatic lawn mower. Moreover, the invention must be sufficiently formed to allow other competent people to make and use the invention.
Using the lawn mowing example, here's a couple of example conversations between an Inventor (I) and a Patent Attorney (PA) to illustrate the difference:
Conversation 1
I: I have a great invention I'd like to patent.
PA: Great, what is it?
I: It's a way to make lawn mowing easier.
PA: Tell me about it.
I: Well, it's an automatic lawn mower.
PA: Sounds like a great idea. How does it work?
I: I'm not exactly sure, but it automatically cuts your lawn like those vacuuming robots, which makes it a lot easier to keep your lawn mowed.
Idea or Invention?
Conversation 2
I: I have a great idea I'd like to patent.
PA: You again. I told you before about ideas and inventions.
I: It's about automatic lawn mowing.
PA: Go away.
I : Seriously, I've been working on this awhile.
...
...
PA: Ok, what have you got.
I: Well, I have attached a steering mechanism to an Arduino module. With my phone, I can set the parameters for the yard. The Arduino module receives the parameters from the phone and then steers the lawn mower according to the parameters. But the module is also connected to several proximity sensors that can detect whether an object is in the way and steer around the object or stop the mower. Since I've also mounted a camera to the mower, I can send a picture to the phone so the user can override the stop command or take direct control of the steering. I've got an app all ready to go and it runs on Andriod enabled phones.
PA: (Jaw Dropped)
Idea or Invention?
If you said Conversation 1 was an idea - You're right! You didn't get fooled by my use of the term, "invention," in the conversation. Nothing gets by you.
If you said Conversation 1 was an invention, you can see why talking with lawyers is no fun at all. Read again and ask yourself, Is this "invention" fully formed or is it just a concept? I think you'll see that not enough details are present to classify this as an invention.
If you said Conversation 2 was an invention - You're right! Again not fooled by my use of the term, "idea," in the conversation. You are probably good at Parcheesi.
If you said Conversation 2 was an idea, you are partially correct. It is an idea that is fully formed, which makes it an invention. Notice the details of implementation - how it works, what are the parts that make the invention operative, and how different situations are handled. I never said this stuff was easy.
Happy Inventing.