You have heard this often, usually as “keep it simple!” But for good designers, just keeping it simple is not enough. If you only just keep things simple, you will still have complicated designs. You must simplify, simplify, simplify!
So what makes a design simple? Can your intuition alone judge simplicity? Will you know it when you see it?
The less thought and the less knowledge a device requires, the simpler it is. This applies equally to its production, testing, and use. Use these criteria—how much thought, how much knowledge—to judge your designs. Judge best by comparing one solution to another. Of course, it may take lots of thought and knowledge to get to a design requiring little of either; that is design.