Can you react to a creature being played at base?

Ruling: You cannot react to a permanent being played. However, you can react to any triggered abilities that the permanent causes when it enters play.

Sequence:

• When a permanent is played, it does not create a chain
• If the permanent triggers an ability, that triggered ability is added to the chain (or starts a new chain if none exists)
• Reactions can be played in response to abilities on the chain, resolving before those abilities

Nuances:

• Reactions resolve before abilities already in the chain
• The distinction is between reacting to the permanent itself (not allowed) versus reacting to triggered abilities caused by the permanent (allowed)