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’s considered to be {0}. Paying a cost changed or reduced by an effect counts as paying the original cost.
May 3, 2019 · War of the Spark
If an effect says that a player can’t gain life, that player can’t make an exchange such that the player’s life total would become higher; in that case, the exchange won’t happen. Similarly, if an effect redistributes life totals, a player can’t receive a new life total such that the player’s life total would become higher. In addition, a cost that involves having that player gain life can’t be paid, and a replacement effect that would replace a life gain event affecting that player won’t do anything.
January 25, 2019 · Ravnica Allegiance
If an effect says that a player can't gain life, that player can't make an exchange such that the player's life total would become higher; in that case, the exchange won't happen. Similarly, if an effect redistributes life totals, a player can't receive a new life total such that the player's life total would become higher. In addition, a cost that involves having that player gain life can't be paid, and a replacement effect that would replace a life gain event affecting that player won't do anything.
January 20, 2017 · Aether Revolt
If an effect says that a player can’t gain life, that player can’t make an exchange such that the player’s life total would become higher; in that case, the exchange won’t happen. Similarly, if an effect redistributes life totals, a player can’t receive a new life total such that the player’s life total would become higher. In addition, a cost that involves having that player gain life can’t be paid, and a replacement effect that would replace a life gain event affecting that player won’t do anything.
February 1, 2014 · Born of the Gods
If an effect says that a player can’t gain life, that player can’t make an exchange such that the player’s life total would become higher; in that case, the exchange won’t happen. In addition, a cost that involves having that player gain life can’t be paid, and a replacement effect that would replace a life gain event affecting that player won’t do anything.
May 1, 2012 · Avacyn Restored
If an effect says that a player can't gain life, that player can't make an exchange such that the player's life total would become higher; in that case, the exchange won't happen. In addition, a cost that involves having that player gain life can't be paid, and a replacement effect that would replace a life gain event affecting that player won't do anything.
September 30, 2011 · Innistrad
If an effect says that a player can't gain life, that player can't exchange life totals with a player who has a higher life total; in that case, the exchange won't happen. In addition, a cost that involves having that player gain life can't be paid, and a replacement effect that would replace a life gain event affecting that player won't do anything.
June 17, 2011 · Commander 2011
If an effect says that a player can’t gain life, that player can’t exchange life totals with a player
April 1, 2011 · Duel Decks: Knights vs. Dragons
If an effect says that a player can't gain life, that player can't exchange life totals with a player who has a higher life total; in that case, the exchange won't happen. In addition, a cost that involves having that player gain life can't be paid, and a replacement effect that would replace a life gain event affecting that player won't do anything.
February 4, 2011 · Mirrodin Besieged
If an effect says that a player can’t gain life, that player can’t exchange life totals with a player who has a higher life total; in that case, the exchange won’t happen. In addition, a cost that involves having that player gain life can’t be paid, and a replacement effect that would replace a life gain event affecting that player won’t do anything.
October 1, 2010 · Scars of Mirrodin
If an effect says that a player can’t gain life, that player can’t exchange life totals with a player
July 16, 2010 · Magic 2011
If an effect says that a player can't gain life, that player can't exchange life totals with a player who has a higher life total; in that case, the exchange won't happen. In addition, a cost that involves having that player gain life can't be paid, and a replacement effect that would replace a life gain event affecting that player won't do anything.
February 1, 2010 · Worldwake
The source of damage is the object that dealt it. If an effect requires a player to choose a source of damage, he or she may choose a permanent; a spell on the stack (including a permanent spell); any object referred to by an object on the stack, by a prevention or replacement effect that's waiting to apply, or by a delayed triggered ability that's waiting to trigger (even if that object is no longer in the zone it used to be in); or, in certain casual variant games, a face-up card in the command zone. A source doesn't need to be capable of dealing damage to be a legal choice. See rule 609.7, "Sources of Damage."