You can't look through your discard pile because some people would slow down the game by repeatedly pawing through it, considering stuff.
Originally you could count it. Valerie didn't like that you had to count it face down, and that we'd have to tell you that. So you can't count it either.
In rare circumstances an effect may try to move a card that is not where that effect expects the card to be. In those cases the card does not move - the effect has "lost track" of the card. Losing track of a card prevents it from being moved, but does not stop anything else from happening.
In our noble efforts to help Dominion become
more insular more precise, as the complexities of the game abound, it becomes necessary to make certain concepts more formal in order to arbitrate certain ambiguities that come up in the game. (The play/in play distinction, the lose track rule, buying vs. gaining, and so on.)
Having familiarity with these concepts/the basic rules of the game makes it pretty easy to play even with cards you have never heard of before. Do everything from top to bottom on the cards you play, and try to win. And even though precision is often important, one does not need to think about what's precisely happening every time a card is played so long as the consistency of the effects with the rules is maintained.
It is on this note that I might call attention to the card Hermit as an example. Hermit is a pretty easy card to use, and fun to play with. You can trash curses that someone else just gave you, and you can maybe gain madmen later. This ease has made the majority of the Dominion community fall into a slumber, one which I now hope to disturb.
What is going on when one plays Hermit?
1. One looks through their discard pile (ordinarily not allowed.)
2. THEN one may trash a card from either their discard pile or their hand.
3. Etc.
Why is step one included? Knowing the contents of one's discard pile is only marginally useful; the whole step seems to be a waste of ink!
Of course, we don't actually think this way. Looking through one's discard pile is essential, because otherwise step two would have strange conditions of fulfillment--one would have to KNOW what is in their discard pile without looking, and the general spirit of the Lose Track Rule seems to be:
one cannot know what one does not see. (What is seen, as a given, in Dominion? One's own hand, the number of cards in one's deck, etc. In addition to this, the
presence of cards in one's discard pile is visible (which is what makes Windfall work), but not the nature of such cards. The top card of the discard pile is weird and we'll talk about it later.)
Once this knowledge is acquired, trashing from the discard becomes a matter of ease.
However, it has already been established that the trashing happens an entire effect
after the looking. The knowledge acquired by looking, it can be assumed, "carries onward", so to speak, through at least the duration of the card's resolution. (It seems
prima facie plausible that this knowledge is limited, however, for the reason that although the contents of one's discard pile usually has been seen in it's entirety, it does not count as "knowledge" in either the theoretical, and even usually the practical sense (unless you've got an eidetic memory).)
What have we established here? That one does not have to be in the process of "looking", in the technical sense, in order for the knowledge obtained from that looking to be able to be utilized by a card.
Let us suppose that we had a Reaction card which reacted to being looked at, much like Patron reacts to being revealed. (The details of this TBD; we can at least assume it would be triggered by Hermit). The Reaction effect would be something like: "put this on top of your deck". There is a question as to, when this card is triggered by Hermit, one can trash a card from one's discard pile. What if the reaction allowed you to put two cards from your discard onto your deck? And so on. There seems to be an ambiguity as to when one's knowledge is sufficiently disturbed in order for a card to play its proper function. Hermit depends on a specific observation of a discard pile in order to carry out its trashing function; if one wanted to be really precise, it seems plausible that any change in the pile in question would prevent the trashing.
This discussion of the interaction of Summon and Death Cart is also of note here. Summon
knows it gained Death Cart, but since two Ruins are sitting on top of it, Summon can't track it and set it aside. Darn!
Here's the fix. Summon should be worded: "Gain a card costing up to $4--but first, watch your discard pile very carefully until this Event is done resolving. Set it aside--carefully. If you did, then at the start of your next turn, play it".
Also, I'm sorry for writing this.
