Zombies are conceivable;- If zombies are conceivable, zombies are possible

 Philosophical zombies are physical and behavioral duplicates of normal conscious humans, without consciousness.  The conceivability argument against materialism runs roughly as follows:

 (1) Zombies are conceivable;

 (2) If zombies are conceivable, zombies are possible; 

(3) If zombies are possible, materialism is false; therefore (4) Materialism is false.