104.1b.

Most of the time, the Magic game uses only positive numbers. You can't choose a negative number, deal negative damage, gain negative life, and so on. However, it's possible for a game value, such as a creature's power, to be less than zero. If a calculation or comparison that would determine the result of an effect needs to use a negative value, it does so. If such a calculation yields a negative number, zero is used instead, unless that effect sets a creature's power or toughness, changes a creature's power or toughness, or sets a player's life total.

Example: If a 3/4 creature gets -5/-0, it's a -2/4 creature. It deals no damage in combat. Its total power and toughness is 2. You'd have to give it +3/+0 to raise its power to 1.

Example: Viridian Joiner is a 1/2 creature that says "{T}: Add an amount of {G} to your mana pool equal to Viridian Joiner's power." An effect gives it -2/-0, then its ability is activated. The ability adds no mana to your mana pool.