Can you use Detonate on nothing but pay the cost anyway?

To determine if you can play Detonate when there are no gears on the battlefield, we must look at its targeting requirements.

Ruling

No, you cannot play Detonate if there are no gears on the battlefield.

According to Rule 355.10, a game object mentioned in the text of a spell is a target unless specific exceptions apply. For Detonate, which reads "Kill a gear. Its controller draws 2," the gear is a target. Because the spell requires a target to be chosen to be played, you must have at least one gear on the battlefield to legally place Detonate on the chain.

Reasoning

1. Targeting Requirement: Detonate's instruction "Kill a gear" targets a gear. As established in Rule 355.10.d.2, even if a gear is the only valid choice, it is still a target.
2. Playing a Spell: To play a spell, you must be able to choose valid targets for all of its instructions that require them. If no gears are present on the battlefield, there are no valid targets, and the spell cannot be played.
3. Comparison to "Up to": If the card text had specified "Kill up to one gear," you would be able to choose zero targets and play the spell without a gear present (as per Rule 355.13). However, because Detonate does not use "up to," it requires a target.
4. Resolution vs. Playing: While Rule 359.3.e.1 states that a spell resolves even if its targets become illegal after it has been played, this does not allow you to initiate the play of a spell without a valid target in the first place.

Summary: You must have at least one gear on the battlefield to play Detonate. If you have a gear, you can play the spell, and if that gear is removed or becomes an illegal target before the spell resolves, the "Kill" instruction will fail, but the "Draw 2" effect will still occur (as they are separate instructions).

Note: I could not fully verify this ruling against the retrieved rules.