Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

Why does short selling require borrowing?
You can't make money on the way down if it was your money that bought the shares when the market was up.  When you sell short, borrowing lets you tap into the value without paying for it. That way, when the price (hopefully) drops you profit from the difference. In your example, if you hadn't paid the £20 in the first place, then you would actually be up £5. But since you started with £20, you still show loss. As others said, borrowing is the definition of selling short. It is also simply the only way the math works. Of course, there is a large risk you must assume to enjoy benefiting from something you do not own!