How did the Enterprise crew learn of Guinan's ability to perceive nonlinear time?

Question

In the ST:TNG episode, Yesterday's Enterprise, Data says that Guinan's species has a perception which goes beyond linear time. Why would Data in the alternate timeline think that?

Back in the main timeline, when Guinan was ripped from the Nexus in "Generations", part of her was still left there. She told Picard she couldn't go back with him because she was already there. The part of her left in the Nexus, a place where time had no meaning, gave her her sixth sense. This explanation was in the original Generations script but was later edited.

GUINAN : It took a long time, but eventually I learned to live with it. And I began to realize that my experience in the Nexus had changed me...I knew things about people...about events...about time...

PICARD : Your "sixth sense"... I've always wondered where it came from

From this, one could think her abilities are not a feature of her species, but of her personally. It seems the Nexus gave her and a handful of other El-Aurian refugees have the ability to perceive time differently.

In addition to the Generations movie, the novel, Engines of Destiny, also makes it clear that Guinan's abilities are due to the Nexus.

Yet, how did the Enterprise crew in the alternate timeline know? Was Guinan involved in some temporal exchange before they encountered the Enterprise-C? And that somehow in that prior event, they became aware of Guinan having an extraordinary power?

If they didn't know, they would never have trusted her advice to send the Enterprise-C crew back through the temporal rift. Please don't tell me that Picard was willing to send them back because one more ship would not have saved the Federation from surrender, but the sacrifice of a single ship long ago would have. He decided to send them back before Data mentioned that the Enterprise-C's sacrifice would convince them to negotiate with the Federation for peace. Picard never would have sacrificed 125 lives solely on intuition. He would have let them live out their days in the current timeline. Unless he knew from experience to trust her.

To be clear, I am not asking why Picard and the Enterprise-D crew sent the Enterprise-C back in time. I am only asking how the Enterprise-D learned of Guinan's abilities.

Answer 1

The whole point of the plot of Yesterday's Enterprise was that the crew didn't know if Guinan was right, but had to do what needed to be done on trust.

As stated in the comments on your question, Data is actually guessing that El-Aurians have time-space perceptions, as a way to explain her perceptions. It's clear during the conversation that the crew are open to the idea that she might have such a power, so it's possible that the altered timeline's past does indeed contain Guinan doing "Guinany" things.

There's multiple factors involved in sending the Enterprise-C back in time:

1. One thing they DO know is that the ship has travelled forward in time.
2. They also know that the Klingons "blame" the Federation for NOT coming to their aid at Niranda 3, one of the many triggers to the war they are fighting.
3. Captain Garrett states, however, that they DID arrive at Narendra III before the temporal anomaly, so the crew can surmise that the reasons the Klingons are angry is because the Enterprise-C was thrown out of time.
4. Picard states that the war is going very badly for the Federation. They're seriously excited about the fact that they might be able to monkey-patch the Enterprise-C into a fighting machine for the new war despite it's already extensive damage. Starfleet expects to have to surrender in less than six months.
5. War is an unnatural state for the Federation. This isn't an alternate timeline where Spock has a beard; it's a timeline with a divergence just a few decades "ago." Picard would be willing to believe that the war they are in is an anomaly.
6. Garrett states that much of her crew wants to go back anyway, as this future doesn't contain their loved ones, and others don't like "sneaking out from a fight." The only reason they'd stay is to fight the Klingons.
7. Picard's closing argument to Garrett that one ship now won't make a difference but a ship at Narendra III might shows that even he thinks it's a long shot, but that it's better than surrender.

So, Guinan's intuition was not the only reason for sending the Enterprise-C back. It was definitely the catalyst, and her absolute conviction that time has changed, and that Tasha Yar should be dead, certainly gave stronger weight to the idea that the Enterprise-C should go back. The episode implies that nobody else had even thought to check to see if the Enterprise-C should be sent back until she mentioned it to them. So her intuition was important in starting the argument.

But, despite the high esteem you give Picard, he most certainly would send 125 lives on a suicide mission if he thought it would save the Federation. As he says to Garrett, the Federation has only 6 months of life left, and the Klingons aren't going to be pleasant masters. That's what convinces Picard, but, remember, he's only a Captain, and so is Garrett (with a few extra decades of seniority.) While technically Starfleet regulations state that, as Captain of the tactically superior ship, Picard could "order" the Enterprise-C back through the rift, it's still up to Garrett to "make it so". Garrett, seeing the state of the Federation and knowing she has a chance to save it, essentially volunteers for the suicide mission.