Relearn

The cleanest possible statement of "buy back your best spell," with no rider attached. Mystical Tutor finds the spell on top of your library; Regrowth crosses into green for any card type; this does the narrow version in mono-blue at sorcery speed and asks nothing in return beyond the three mana. That bluntness is the design point. Where later recursion sorceries tried to earn their slot with upside (a scry, a discount, a second target), this is the bare template the others were measured against: take one instant or sorcery from the yard, put it in hand, done. The strategic axis it sits on is card advantage stretched across turns rather than tempo within one: you pay full retail to re-cast a spell you already paid for, so the payoff scales with how much the recurred spell is worth rather than with the recursion itself. That makes it a tool for decks that have already broken the symmetry of "one spell, one effect," counterspell shells that want to re-arm, combo lines that need a specific piece back. On a fair-rate spell it is just clunky. On a spell that wins the game or locks the opponent out, it is a second copy on demand, which is the whole reason the effect keeps getting reprinted in narrower, splashier forms while the original stays the reference point.