Pick up cards until you pick up a monster that can be Normal Summoned.
You can Special Summon a high-level monster like "Dark Magician" with "Reasoning", but not Special Summon-only monsters like "Gate Guardian", Ritual Monsters, etc.
If you pick up a Spirit Monster, then all picked up cards are sent to the Graveyard.
If no monster is picked up, shuffle your Deck and put it back in the Deck Zone; the effect of "Reasoning" disappears.
You cannot activate "Reasoning" if you already activated "Scapegoat" this turn.
If "Banisher of the Light" is Special Summoned with "Reasoning", the other cards picked up for "Reasoning" are sent to the Graveyard. The effect of "Banisher of the Light" is not applied until after "Reasoning" resolves.