Which player has priority is determined by the following rules:
May 3, 2019 · War of the Spark
A player can’t pay a cost without having 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 202, “Mana Cost and Color,” and rule 602, “Activating Activated Abilities.”
January 25, 2019 · Ravnica Allegiance
A player can't pay a cost without having 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 202, "Mana Cost and Color," and rule 602, "Activating Activated Abilities."
April 27, 2018 · Dominaria
A player can’t pay a cost without having 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 202, “Mana Cost and Color,” and rule 602, “Activating Activated Abilities.”
May 1, 2014 · Journey into Nyx
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 202, “Mana Cost and Color,” and rule 602, “Activating Activated Abilities.”
February 1, 2014 · Born of the Gods
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 202, "Mana Cost and Color," and rule 602, "Activating Activated Abilities."
September 30, 2011 · Innistrad
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 202, "Mana Cost and Color," and rule 602, "Activating Activated Abilities."
June 17, 2011 · Commander 2011
A player can’t pay a cost unless he or she has the necessary resources to pay it fully. For
April 1, 2011 · Duel Decks: Knights vs. Dragons
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 202, "Mana Cost and Color," and rule 602, "Activating Activated Abilities."
February 4, 2011 · Mirrodin Besieged
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 202, “Mana Cost and Color,” and rule 602, “Activating Activated Abilities.”
October 1, 2010 · Scars of Mirrodin
A player can’t pay a cost unless he or she has the necessary resources to pay it fully. For
July 16, 2010 · Magic 2011
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 202, "Mana Cost and Color," and rule 602, "Activating Activated Abilities."
February 1, 2010 · Worldwake
If an effect causes a player to gain life or lose life, that player's life total is adjusted accordingly.