An asset is exactly something that can generate future cashflows, even if only by being sold. A security is a special kind of asset that can be easily traded on paper without changing it's value or taking on transformative effort.

So, your goose that lays golden eggs is indeed an asset, but it might be hard to trade because many folks aren't well prepared to own a giose. If you wrap it up in contracts about someone who will house and feed the goose, collect and sell the eggs and forward the money to the holder of a deed - congrats, that deed is now a security.