Arcane Subtraction

A -4/-0 debuff answers a narrower question than most combat tricks: it does not kill the attacker, it defangs it. The creature still connects, still blocks, still sits on the battlefield; it just cannot push meaningful damage through this turn. That makes this a defensive tool priced for tempo, a way to blunt an alpha strike or make a favorable block without spending a card on removal that leaves you down. The Learn clause is what changes the math. A pure -4/-0 spell is a one-shot with no floor: if the board does not cooperate, it is a dead card. Attaching Learn hands it a rider that usually pays off, either fetching a Lesson from your sideboard or, if you have a card to spare, a rummage that trades one into a fresh card. The instant-speed window matters too: you hold it up, decide in the attack step whether the -4/-0 line is live, and even when it is not the sharpest use you can still cash the Learn for filtering as the spell resolves. That is the design idea behind pairing a conditional effect with a guaranteed one; the conditional part can be pointed and cheap because the card is not asking you to bet a whole slot on the board state breaking your way.