I make assumptions to save time. It's no use to question every detail of everything. Assumptions are not 100% accurate, but they are accurate enough to keep you on the track.
But then again, sometimes assumptions can be dreadfully wrong, and lead to catastrophic results.
What's the point of this topic? I've failed to see it, and I think without a point this thread will be nothing but empty words of how everything can be anything and we can never be sure about anything. So, to build an interesting discussion here, we need more stuff! Examples, claims, statements, anything!