"[Primary source] means "a source of the first order of importance." That specific meaning is well understood among historians. Knell perhaps did misunderstand its meaning, not being a historian."
I'm never quite sure which irritates me more: the fact that so many of Welsh's statements are utter rubbish or the smug conviction used to state them. At any rate, as usual, he is quite wrong.
Since other people may also be unfamiliar with the historiographical phrase, I'll try to clarify it ...
I suspect Welsh is confused by the word "primary", looking no further than its popular and commonest definition as "being of chief importance", whereas in this case the word is meant in its more academic and rarer alternative definition as "being earliest in time".
While such evidence would normally rank first in order of preference to historians seeking reliable material, the word "primary" in the phrase has nothing to do with that rank. The word refers solely to the fact that, unlike a secondary source, the evidence is as close to the matter being examined as possible.
Nor does the phrase necessarily correlate to the "order of importance" in the composition of a work. A scholar compiling a history of, say, a town may find that by far the most important source for his narrative is earlier published books but, unless they are first-hand accounts, they are unlikely to be a "primary source" - whereas a tatty old deed, verifying a minor event so incidental that it warrants only a footnote, would be.
The sole criterion for "primary source" is the degree of originality, not importance. The fact that the source is original, and therefore more reliable, may well be of huge importance - provided that it imparts information of value - but that importance is not what defines it as "primary".
The real meaning of the phrase is indeed "well understood among historians". To turn the snide comment made by the owner of Classical Coins back on him: Welsh certainly did misunderstand its meaning, not being a person in the habit of bothering to check facts before making statements.
Sadly, it seems he knows as much about historiography as he does about archaeology. And the blindingly obvious fact that archaeological materials provide a major source of original evidence highlights the insanity of his remark that "archaeology is not a primary source for the development of history".
Both archaeology and numismatics can provide a "primary source" for historians. It's not some kind of bizarre competition - but I'll explore that fantasy contest in another post ...