There is the rule, when ordered to discard with no source given then you discard card(s) from your hand.
Why does anything else — play, reveal, trash, … — need the "from your hand"?
I mean, Dominion is a card game, in which by common sense alone you use the cards you have in hand – if not stated otherwise by rules (or Dominion card texts). So why not expand the discard rule to general rule saying any action a card text orders to do with another card is meant to be done to a card from hand if no other location is given or the card in an other location is referenced anyhow?
So instead of Throne Room's "You may play an Action card from your hand twice." it would be sufficent to read "You may play an Action card twice." while anywhere else explicit location gets written anyway (Vassal the card it discarded, Wine Merchant the Tavern mat, …)
