I think the issue is precisely the rationalising part.

Let's assume I have pre-decided something: if the fruit weighs over 100g, I'll buy it, otherwise I won't.

I can then measure and find out. My emotions -- based on looking at the fruit -- may mislead me here.

Now what about "likely to weigh more than 100g"?

Here it's really easy to lie to yourself.

But if you set conditions for what counts as likely before you know what they'll be used for, and you stick to them, you can make a decision based on reason.

There is no way to decide 100g is the threshold divorced from emotion, however -- unless it's rationally derived from something else emotionally based.

The "rationalisation" bit comes from us not making decisions for the reasons we think we are -- or in the way we claim to be.