"Unsubstantiated reasoning"

So often people don't justify things they say. I had aways explained this situation in those terms: that the person isn't providing any justification, that they're essentially just asserting that their belief is true. And while I still think that's a technically correct description, it's always been problematic in getting the point across.

Problematic because the person might have given some justification for some of their points, even if those justifications are simply a way of deferring their assertion - i.e. they are something that equally requires justification. I've been aware of this problem, but not known how do better.

It occurred to me that a useful term might be "unsubstantiated reasoning". Someone may present their reasoning, but they haven't substantiated it - there's no reason to belive why the premises hold, or that the reasoning holds, or is relevant, in this situation.

[A search on Google tells me that there are 29 pages (or 29 relevant pages - 73 if you count lots of similar pages) that use this phrase]