G.19.1- Each player starts the game with 20 life points. [CompRules 1999/04/23]
G.19.2- A player may gain life points to have a higher total during the game. There is no upper limit. [D'Angelo 1998/02/03]
G.19.3- A player loses the game if they have less than 1 life. This is a state-based effect (see Rule T.11). [CompRules 1999/04/23]
G.19.4- All players are entitled to know each other's life totals at all times. [D'Angelo 1998/02/03]
G.19.5- A spell or ability may reduce a player to less than zero life by means of damage or loss of life. [D'Angelo 1998/02/03] This rarely matters since a player normally loses when this happens, but an effect may prevent the player from losing due to a life total of 0 or less.
G.19.6- If a spell or ability allows you to pay life points to get some effect, you cannot pay more life points than you have. If you already have zero or less life points, you may not pay for any spell or ability which requires a payment or 1 or more life points. [CompRules 1999/04/23]
G.19.7- A player at zero or less life is treated as having zero life for all reasons other than raising their life total. [D'Angelo 1998/02/03] This rarely matters since a player normally loses when this happens, but an effect may prevent the player from losing due to a life total of 0 or less.
G.19.Ruling.1- Because of Rule G.19.7, if you are instructed to lose a fraction of your life points while you are already below zero life, your life total does not change. [D'Angelo 1998/02/03]
Note- Also see Loss of Life, Rule G.21.
Note- Vanguard cards can modify your starting life total, see Rule E.14.