A transaction is a completed agreement between a buyer and a seller to exchange goods, services, or financial assets in return for money.
Yes. If both of us agree that Bitcoin is money, then we can start exchanging stuff for bitcoins.
Where's good, where's service, money, asset... if is the only thing that's going on is the change of numbers next to addresses?
You give me food, I give you bitcoins, so there's the food. Same happens with every currency, why shouldn't it happen with Bitcoin?
The only thing we can agree on is for numbers to change next to our address. If I give you food, and the numbers are changed, where's bitcoin in that transaction? The only transaction is me giving you food. You gave me nothing. So, I can repeat the question: