Creativity: How do you know when a project is finished?

How do you know you have said what you wish to say?

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One of my piano students asked me this question some time ago: how do you know when a project is finished? I thought it was an excellent question, and also a very difficult one. To my knowledge, I have never been overtly taught this, nor have I heard a definite answer from history. Truthfully, I wonder if anyone can