Can you use Not So Fast to counter an opponent's Not So Fast that is targeting your unit's play effect?

Ruling: No, you cannot use Not So Fast to counter another Not So Fast. Not So Fast targets spells and abilities, not units or gear, so it cannot be a legal target for another Not So Fast.

Sequence:

• Attacker attacks
• Defender plays Janna (unit enters battlefield)
• Janna's play effect triggers and targets attacker's unit
• Attacker plays Not So Fast targeting Janna's play effect
• Defender cannot play Not So Fast to counter the attacker's Not So Fast

Nuances:

• When Not So Fast counters a unit's play effect, the unit still enters the battlefield; only the ability is countered
• When countering an ability with multiple effects (like Janna's heal and target), the entire ability is countered, not just one part