Hello Fire,
For the most part that may be correct. Our faculty of reason is not a pair of eyes that is able to detect moral properties, but instead we have to make representations to it - that is, we have to, so to speak, describe to it how things appear to us to be - and then it tells us (or does if it is operating well) what moral features are present and what it would be right or wrong to do in the situation we have described. In this sense, our conscience - which I take just to be the name we give to our reason when it is telling us about moral features - is held hostage to information we provide to it.
But I think it must be admitted by all that our faculty of reason contains important information about reality, otherwise consulting it would tell us nothing about anything.
Imagine there's a guide book to a jungle. This guide book warns about eating certain sorts of berry - perhaps it says to steer clear of eating any yellow berry, or any yellow berry above a certain size. It does not say anything else about the berry, and it does not tell you where specifically these berries are. So one could not use the guide book to find the berries. Nevertheless, if provides one with important information: it warns against eating any such berries one may come across.
It is reasonable to infer from this warning in the guide book that any yellow berries one comes across in the jungle are poisonous, or likely poisonous.
The guide book is analogous to our faculty of reason, the warning is analogous to our reason telling us not to do something, and the poisonousness - or likely poisonousness - of the berries is the fetus's person status. If our faculty of reason - or at least, the faculty of reason of many - warns us against abortions, then it is reasonable to infer from this that the fetus has a mind, as this is the best explanation of why it is warning us against having them if, that is, this is what it does.
On the other hand, if it issues no such warning - or only issues it if one represents the fetus to b a person (which would be equivalent to looking up 'should I eat poisonous berries?' in the guide - a question that it will obviously answer with 'yes' and that tells one nothing about whether the yellow berries are poisonous or not) - then it is reasonable to infer that the fetus is not a person.
I don't think there's a problem with making such an inference. It seems to me no less problematic than inferring that the yellow berries the guide book is warning us against eating are poisonous (given this seems the best explanation of why we are being warned against eating them). And we do still have to provide information to our reason: we have to describe the scenario. And then it delivers its verdict. That's equivalent, as I see it, to seeing some yellow berries and then looking up 'yellow berries' in the guide book and seeing a warning against eating them (and then inferring from this that they are poisonous). But most people aren't doing this, I suspect, and are instead looking up 'poisonous berries' and seeing 'don't eat poisonous berries' or looking up 'non-poisonous berries' and seeing 'it's fine to eat non-poisonous berries'. That is, they are either asking their reason 'is it okay to kill a little person who is inside of one?' or they are asking their reason 'is it okay to destroy a lifeless lump of cells that is inside of one?'. Obviously the reason of the first group says loud and clear 'no', and the reason of the second says equally loudly and clearly 'yes'. And if either side asked the other side's question, they'd get the other side's answer. Hence why the classic debate is deadlocked. Neither side is really wrong, given the questions they're asking. But they seem to me to be going about things in teh wrong way....