Conjecture and Opinion

I've gone on a bit about facts and assumptions, what they are and how they're used.  But consider also what they're not.

An assumption takes the place of a fact you don't actually have.  You think it's true, and you're confident that is it probably true -- you just don't know for sure.  An assumption is as strong as the actual facts it is assembled from or based on, and you need to understand that basis to decide just how to use the assumption.

For example, during a conventional war soldiers might spot a self-propelled artillery piece on the battlefield.  You know that enemy doctrine always groups these into batteries of four, and that they are deployed only with an independent tank regiment, which in turn operates only as part of a specialized division.  Because you know enemy doctrine, one fact allows you to deduce all the other facts.

But then come the assumptions: that the artillery piece is operational, that it has a trained crew, that it has ammunition and fuel.  How strong are these assumptions?  You need more information.  And depending on that additional information, you may have greater or lesser concern about the presence of that one gun out on the battlefield.

If the facts are not so certain, if, to use this example, there were no published doctrine on the organization and deployment of self propelled artillery systems, then any statement you might make about the units out there would be guesswork, perhaps reflecting how you would run the enemy's army.  But it wouldn't be very strong.  And if your soldiers didn't actually get a good look at the gun, or weren't particularly well-versed in how to recognized various artillery pieces, then even the observation itself might be pretty weak.

At some point, if the solid facts lack sustance, then you can't make a valid assumption at all.  If you insist on making a claim anyway, it's really nothing more than conjecture -- a guess, a hunch, something that may be more imagination than real, based more on instinct than observation.

Conjecture is not a good thing to build on.