Jim, I think this is where I am trying to go. It is very difficult to separate my confidence in my belief that I am right from my confidence that I have discovered the actual cause (and you're right, it is difficult to follow sometimes.) If I (hypothetically) am honest, there are times when I am not confident in either and I call those "undetermined". There are times when I am certain that I have the true cause and I can call that the probable cause or a scientific certainty - it means the same to me; that I am very confident that I am making the right decision. But whatever words I use, I must support that belief with some observations made at the scene in question, filtered through current, validated scientific knowledge. The issue, as I see it, is that the adversarial system deliberately searches for alternate interpretations of data; and usually finds it. You and, say, Pat, disagree because you are both confident in your opinions as to the cause. You are both reasonable professionals (most of the time) and have based your opinions on good methodology - does fancy wording make one of you more certain, or more correct, than the other?

It seems that any attempt to label an absolute level of certainty becomes an infinitely regressing argument that is ultimately inconclusive.

Let's look at the two options regarding "degree of certainty" that you alluded to:
1) How confident am I that I got it right and/or
2) How confident would an independent fact finder be that I have revealed the "true" cause?
If you ask me how confident am I that I got it right? To a degree of scientific certainty.
If you ask the opposing expert how confident they are in their interpretation of the data? To a degree of scientific certainty.
One (or both) of you has misapplied the use of the term "scientific certainty". To know which of you is right, we would have to know the "true" cause of the fire.

As you said, I wouldn't list the cause if I wasn't confident that I was getting it right based on the data available to me at the time the decision was made. (I reserve the right to alter my certainty based on new data that was previously unavailable.)

There is no doubt that origin and cause determinations should be based on a professional methodology and thoughtful evaluation of data; and no doubt that the investigator should be confident that they are getting it right, but using emphatic words does not make it so. A jury may be impressed with an expert who is supremely confident in their own opinion, but they could still be wrong.