How can Vader not see that Leia is his daughter?

[B-LoVeD AssAsSiN]

I mean look how many times he confronted her he was so strong in the force he or palps could easily dispel the deception seeded by obi wan or Vader could have at least seen that she was so strong in the force.

[JMK]

He didn't know he had children, or so it's rumored at this point. As to why he didn't feel her presence as his daughter, that's debatable. He felt Luke, but not Leia. Could it be that Luke had begun dabbling in the force just prior to the assault on the death star so it made his presence more easily felt.

Or, when writing ANH, Lucas didn't plan on making Luke and Leia siblings, so there would have been no reason for Vader to sense Leia.

[Doc Milo]

Let's use this bit of imagery. There is a running river. One person stands still in that river. The water rushes around his feet. Another person swims in that river, splashing around. The force user can only sense the movement of the water. So, he/she doesn't sense the person standing still (Leia, not actively using the Force) but he can sense the ripples and splashes in the water from the person swimming (Luke, actively using the Force.)

And . . . as JMK said, he didn't know he had any children, so there is no reason why he'd try to sense one of his children. Who says that a Force user can sense anyone without actively trying to sense that person, or without previously knowing that person? What if people have their own "signitures" in the Force and a user has to know that person to know his signiture, and thus sense that person specifically? This is sort of supported. In TPM they sense a "disturbance" which is Anakin, as well as the manipulations of the Sith -- but they don't know the source of the disturbance. In ANH, Vader senses the presence of his old master. Someone he previously knew.

[CMJ]

Doc, I have to agree with what Carr said in another thread. None of us can explain SW like you can....you're a helluva a read. :)

[Jedieb]

Or, when writing ANH, Lucas didn't plan on making Luke and Leia siblings, so there would have been no reason for Vader to sense Leia.
That's it. As late as the writing of the script for ROTJ it was still undecided as to who the "other" Yoda spoke of was. I think if Lucas had Leia's parentage in place at the time of ANH he never would have had that torture scene. (He sure as hell wouldn't have had Luke hitting on her so hard or their kiss.) Think of this, Anakin used the Force as a child whenever he was podracing. It was the only reason he was the only human that had fast enough reflexes to pilot podracers. Qui-Gon labled it as a Jedi trait. So you can access some of your Force potential even without training. Think of what inner strengths Leia had to draw upon to resist Vader and the mind probe. How could she NOT have drawn upon some of those strenghts?! How could Vader not sense ANY of her strenghts? It doesn't make sense, because she WASN'T his daughter at the time. But, now we have to backtrack and try to make it work. Doc's explanation is as good as any. Although I think Leia wasn't simply standing in a river. She was "splashing" around when she resisted that mind probe and Vader's torture. At least that would be my take.

[JMK]

True too. Anakin wasn't exactly actively using the force, or at least he didn't know he was. Both Qui Gon and Obi Wan sensed him though. In the same train of thought, Vader should have been going nuts at the beginning of ANH when he flew over Tatooine. His "daughter" is on the ship, and his son is on the planet's surface. He didn't realize a thing. But then he senses Luke at the end of ANH, once Luke acknowledges that he's now using the force. Is it like turning a switch on, and exposing yourself, similar to what Doc is saying? SW lore contradicts itself a little with this. Anakin is detected from afar even before he willingly uses the force. But Luke and Leia are not. Could it be as simple as Anakin having SO many midichlorians that there's no way any Jedi would not sense him? That would be an easy way out for Lucas, but personally I would like to hear something a little more dignified and plausible.

[Doc Milo]

"I've encountered a virgence in the Force."

Perhaps the "disturbance" both he and Obi-Wan feel when they first land on Tatooine isn't Anakin at all -- or isn't merely Anakin's unconcious use of the Force. Perhaps unconcious use of the Force is not enough to sense anything. Perhaps the only reason they sense a disturbance is because of the virgence in the Force that is centered around Anakin....

[Syren]

Ok, this might be far out in left field, but I'd like to put in a possible twist or idea on this still keeping in mind that Anakin/Vader isn't aware he has fathered twin force-users.

We all know that in the end, Vader becomes Anakin again when he tosses the Emperor down that shaft to save his son from getting fried to a crispy critter by the force lightning.

Well, I had wondered why Vader didn't just off Leia when he had the chance when boarding her ship as she had the plans to the deathstar. I know that the movie will tell you the obvious answers to that question: He wanted the plans back, plus he wanted to torture her by forcing her to watch the destruction of her home planet of Alderaan. Ok, well thats the obvious. What about the not so obvious?

Such as, there were plenty of times when Vader very easily could have offed Luke and Leia, but he doesn't. Why? Maybe, just maybe, on some deep subconcious level, he had an inkling of these two possibly being of his blood? Some feeling that just nagged in the back of his mind telling him not to kill them? I'd like to think that is another possible reason. Could a mother, or father for that matter, in the real world .. even after being separated from their offspring for what ... over twenty years, really NOT know somewhere deep down that they are looking into their own eyes??

Maybe the moment Vader very first sees Luke and Leia, a part of him somehow knows. Maybe he sees his own eyes and his own younger, former self in Luke and somehow he feels a need to preserve (or even taint) that.

Just a thought.

[JMK]

As evil as Vader was, I don't know if he had it in him to kill women. As for Luke, he would have blown him to bits in the Death Star trench had Han Solo not intervened.

[Doc Milo]

JMK: "I killed them. I killed them all. Not just the men. But the women. And the children. They're animals, and I slaughtered them like animals. I hate them!"

I don't think he would have drawn the line, as Vader, at killing women.

He doesn't kill Leia because she has information that she wants. When she proves resistant to telling him that information, Tarkin orders her termination. Vader doesn't plea for her life.

After that -- the only other time when they are in Vader's clutches is on Bespin, and her use there was to draw Luke to him by torturing both her and Han so that Luke would feel their pain through the Force and come running. Other than that, I don't see anywhere else in the saga where he had the opportunity to "off" Leia. As for Luke, in ANH he's never come face to face with Luke, really, their only "confrontation" was in the trench, and as JMK said, Vader would have blasted him out of space if Han had not intervened. In ESB, he knew who Luke was, and instead of trying to kill him, was trying to turn him. In RotJ, he brought Luke to the Emperor so they could both try and turn him. I don't see all these opportunities for Vader to have "offed" either of them...

[JMK]

Yeah, I forgot about that Episode with the Tuskens.
And don't believe Vader when he tells the Emperor "He will join us or die, Master." I don't think at that point he had any intentions of killing Luke, but was bent on turning him to the dark side no matter what.

[Oriadin]

Luke and Leia are twins. Perhaps he didnt realise he had two children and only realised he had Luke.

I belive that Vader can sense when people are strong in the force and he can sense familiar people. I dont know if there is any evidence of the force being used to tell if someone is a relation.

[Doc Milo]

There is no evidence to suggest that Vader knew he had any children at all, before finding out the identity of the pilot who destroyed the first Death Star some time between ANH and ESB.

In fact, it makes more sense that he didn't know about any children, because then the "hiding in plain sight" of Luke on Tatooine, Anakin's home planet, without changing Luke's last name, has at least some explanation as to why Vader didn't discover Luke before that point in time.

If he knew he had a child -- he and the Emperor would have searched the galaxy to find the child, no matter the place or cost -- because the child would be a threat to them. (That's the reasoning behind Obi-Wan and Padme hiding the children.)

Before the time when Vader discovers the identity of the pilot strong in the Force that destroyed the first Death Star, I don't believe he knew he had any children at all.

I think the best explanation is that the unconcious use of the Force that Leia might have been doing was so miniscule that no Force user could have detected it as a use of the Force. She might not have even used the Force unconciously -- the Force might have made her mental defenses stronger on its own, making her ability to resist the mind probe "considerable." I think that to sense a specific person in the Force, the best explanation is that the person would have had to have been known by the Force user.