Now we are talking about validating input, even though I just claimed that it's not a responsibility of a use case class. I still think, however, that it belongs to the application layer, so this is the place to discuss it.
Why not let the calling adapter validate the input before sending it to the use case? Well, do we want to trust the caller to have validated everything as is needed for the use case? Also, the use case might be called by more than one adapter, so the validation would have to be implemented by each adapter and we might get it wrong or forget it altogether.
The application layer should care about input validation because, well, otherwise it might get invalid input from outside the application core, and this might cause damage to the state of our model.
But where to put the input validation if not in the use case class?
We will let the input model take care of it. For the "Send Money" use case, the input model is the SendMoneyCommand class we...