A Harvard philosopher named Michael Sandel came up with a "thought exercise" to prove that we don't really believe that life begins at conception. It goes like this:
You are in a fertility clinic and there's a fire. On your way out you hear a noise and check in a room. You find a five year old child and a container marked "1,000 embryos". The fire and smoke are getting bad and you can only save one. Do you save the child or the container of embryos? Naturally you will save the child which means that you don't really believe the embryos are people which, in turn, means that you've been lying in order to contain control of women's bodies.
I cleaned it up a bit but that's what he says. You can see the whole thing laid out here if you really want along with a separate take-down.
So, does this thought exercise do what it claims to? Not really. It was contrived to make you choose the desired outcome. There are several reasons that the average person would choose the child. Here are some of them:
1) We don't handle abstracts well in a crisis. Most people wouldn't even stop to read the containers. They'd grab the child and run.
2) We are hard-wired to choose the concrete over the abstract. We see this constantly in movies and TV shows - someone is given a choice to save a hostage knowing that it could mean the death of many others. Given the choice between a live child and a container, people will choose the child, even if they know that the container represents more children. What's more, we don't actually know what the contents of the container are. Just because it has a label does not mean that it is currently full or even in use.
3) We know more than we are told. Anyone who knows how in-vitro fertilization works knows that we are already in squishy grounds. Only a fraction of the embryos will actually be successfully implanted so it is not a 1000 vs 1 choice. Most of these embryos will be discarded or die in failed implantations (which is why the whole process is morally squishy). The catholic Church debated allowing this process for this very reason.
4) This is an impossible situation. Embryos are not freeze-dried, ready to add water to reconstitute. They are kept frozen at near absolute zero. So the container in question would actually be a larger freezer. Disconnecting it would lead to the death of the embryos within minutes. To put this in perspective, let's take an alternate thought exercise. You are in a maternity ward and a fire breaks out. You see a five year old and two premature infants in incubators. Do you save the child or do you save the two infants, even knowing that they will die unless they can quickly be put back in incubators? And, if you choose the child does this mean that you do not think that the infants are actual people until they can breath unassisted?
It's one thing to propose a thought exercise to make people clarify and justify their reasoning. But that is not what Sandel did. He is playing a mind game to enforce his point of view. And he's not doing a very good job of it. He needed to propose it in neutral terms so that people's defenses were not raised. But he is really just performing for the people inside the bubble. He's not expecting anyone with dissenting views to really examine his exercise.