General Rulings - 424

• 424.1 - A cost is an action or payment necessary to take another action or to stop another action from taking place. [CompRules 2008/02/01]
• 424.2 - A player can't pay a cost unless he or she has the necessary resources to pay it fully. For example, a player with only 1 life can't pay a cost of 2 life, and a permanent that's already tapped can't be tapped to pay a cost. See Rule 203, "Mana Cost and Color," and Rule 403, "Activated Abilities." [CompRules 2007/05/01]
• 424.3 - Unpayable costs can't be paid. (See Rule 203.1b.) [CompRules 2007/05/01]
• 424.4 - What a player actually needs to do to pay a cost may be changed or reduced by effects. If the mana component of a cost is reduced to nothing by cost reduction effects, it is considered to be {0}. Paying a cost changed or reduced by an effect counts as paying the original cost. [CompRules 2007/07/13]
  Example: You play a spell with mana cost {W} that has kicker {1}. You choose to pay the kicker, but a cost reduction effect means you spend only {W} when paying for the spell. The spell's "if the kicker cost was paid" effect will be applied. [CompRules 2007/05/01]
• 424.5 - If a cost includes a mana payment, the player paying the cost has a chance to play mana abilities. Paying the cost to play a spell or activated ability follows the steps in Rule 409.1f through Rule 409.1h. [CompRules 2007/05/01]
• 424.6 - Some spells, activated abilities, and triggered abilities read, "[X]. If [a player] [does or doesn't], [effect]." or "[a player] may [X]. If [that player] [does or doesn't], [effect]." The action [X] is a cost, paid when the spell or ability resolves. The "If [a player] [does or doesn't]" clause checks whether the player chose to pay an optional cost or started to pay a mandatory cost, regardless of what events actually occurred. [CompRules 2008/10/01]
  Example: You control Hesitation, an enchantment that says "When a spell is played, sacrifice Hesitation. If you do, counter that spell." A spell is played, causing Hesitation's ability to trigger. Then an ability is played that removes Hesitation from the game. When Hesitation's ability resolves, you're unable to pay the "sacrifice Hesitation" cost. The spell is not countered. [CompRules 2008/10/01]
  Example: Your opponent has played Gather Specimens, a spell that says "If a creature would come into play under an opponent's control this turn, it comes into play under your control instead." You control a face-down Dermoplasm, a creature with morph that says "When Dermoplasm is turned face up, you may put a creature card with morph from your hand into play face up. If you do, return Dermoplasm to its owner's hand." You turn Dermoplasm face up, and you choose to put a creature card with morph from your hand into play. Due to Gather Specimens, it comes into play under your opponent's control instead of yours. However, since you chose to pay the cost, Dermoplasm is still returned to its owner's hand. [CompRules 2008/10/01]