Why can't Blue Sentinel's power be used to pay for Power Nexus's optional cost?

Question

Blue Sentinel gives you power at the start of your next main phase when you hold a battlefield with it, while Power Nexus lets you optionally pay power as part of an ability that resolves during the beginning phase's hold triggers. A player wondered why they couldn't use the power from Blue Sentinel to pay Power Nexus's cost, since a "you may pay X to do Y" cost is normally paid on finalization rather than immediately when the ability is put on the chain. Why can't Blue Sentinel's power be used for Power Nexus's cost?

Anonyme · Accepted Answer

Even though a "you may pay X to do Y" cost is paid on finalization rather than when the ability goes on the chain, that still happens during the beginning phase, while Power Nexus's hold trigger is resolving. Blue Sentinel's card text specifically gives you its power only at the start of your next main phase, which comes after the beginning, channel, and draw phases — so you simply don't have that power yet when it's time to pay Power Nexus's cost.

Why can't Blue Sentinel's power be used to pay for Power Nexus's optional cost?

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