Can you pay life as an alternative cost if you don't have enough life to pay it?

Paying life is a cost, and costs must be fully payable at the time you activate an ability or cast a spell. Under CR 117.3, a cost that requires paying life can only be paid if you have enough life to cover it — you cannot pay life you don't have.

However, there is a nuance: you can pay life that would reduce your life total to exactly 0. Reducing your life total to 0 does not mean you lose the game at that instant; state-based actions (CR 704.5a) check after the spell or ability finishes resolving. The payment itself is legal as long as you have that much life to spend.

So the practical rule is: you can pay a life cost as long as you currently have at least that many life points. If the cost is greater than your current life total, you cannot pay it and therefore cannot cast the spell or activate the ability at all (CR 601.2h requires all costs to be payable).

Example: You are at 2 life and want to cast a spell with an alternative cost of paying 3 life. You cannot cast it using that alternative cost because you only have 2 life — you are 1 short of the required payment, making the cost unpayable.

Certain cards like Platinum Emperion prevent your life total from changing, which would also make any life-payment costs impossible to pay while that card's effect is active, since the payment would be unable to actually reduce your life total (CR 119.4).