Right, and to add upon this - that's because all ideas originate from the senses.
To demonstrate: try to imagine a new color you've never seen before that's not in any way associated to any of the colors that you've seen. (it's impossible)
Or think of it this way: You could explain a car to someone who has never seen a car before, but only so long as the ideas used to explain the idea of a car (e.g. wheels, doors, windows), have already been familiarized to the other person. Otherwise if those ideas weren't familiar, such as a wheel, you'd need to also explain what a wheel is. And if the concepts used to explain the wheel wasn't familiar (and so on), you'd eventually hit a point where you must expose the idea(s) directly to their senses (eg show them), otherwise they will never understand what you're talking about.
So all ideas come from the senses, and your minds ability to combine these "pure" ideas that you've sensed. "Pure" ideas are cheap and easy because they're the simplest ideas - they're what you directly sensed. To have good ideas, you need to combine many "pure" ideas together, hence why those who have experience working closely and thoroughly with something, will often have the best ideas associated to that something...