This is an excerpt from an article on the Infidels website. Often the charge is leveled against revisionists that their arguements are poor and largely Arguements from Silence, that is that there is no evidence FOR something therefore it is not true. This is a valid method when used responsibly. The following attempts to explain as much.
The Argument from Silence
Evangelical apologist Craig Blomberg argues that one should approach all
texts with complete trust unless you have a specific reason to doubt
what they say (The Historical Reliability of the Gospels, 1987, pp.
240-54). No real historian is so naive (see Bibliography ). I am not
aware of any ancient work that is regarded as completely reliable. A
reason always exists to doubt any historical claim. Historians begin
with suspicion no matter what text they are consulting, and adjust that
initial degree of doubt according to several factors, including genre,
the established laurels of the author, evidence of honest and reliable
methodology, bias, the nature of the claim (whether it is a usual or
unusual event or detail, etc.), and so on. Historians have so
much experience in finding texts false, and in knowing all the ways they
can be false, they know it would be folly to trust anything handed to
them without being able to make a positive case for that trust. This is
why few major historical arguments stand on a single source or piece of
evidence: the implicit distrust of texts entails that belief in any
nontrivial historical claim must be based on a whole array of evidence
and argument. So it is no coincidence that this is what you get in
serious historical scholarship.
Even so, there is nothing inherently dubious in the claim that Jesus
existed. So there is no need for much evidence to ground a reasonable
belief that he did, so long as that evidence can be trusted more than it
can be doubted. However, when trust and doubt are in balance over all
the existing evidence, an Argument from Silence can tip the scales. ......
But if a
significant Argument from Silence (AfS) can be made, then all else is
not equal. There would then be an increase in the probability of "an
unexplained legend of so uncommon a type in so short a time," and enough
of an increase can overcome the prior probability of a real Jesus.
How does one make a good AfS? Gilbert Garraghan explains:
To be valid, the argument from silence must fulfill two conditions: the
writer[s] whose silence is invoked in proof of the non-reality of an
alleged fact, would certainly have known about it had it been a fact;
[and] knowing it, he would under the circumstances certainly have made
mention of it. When these two conditions are fulfilled, the argument
from silence proves its point with moral certainty. (§ 149a)
This is a slam dunk AfS. But an AfS can be deployed that is relatively
weaker to the extent that either condition is less certain. That is, it
may only be "somewhat certain" that the relevant authors knew x and
would mention it, and in this case the AfS only produces a less than
"somewhat certain" conclusion. In general, based on the hypothesized
entity itself, and in conjunction with everything we know on abundant,
reliable evidence, should we expect to have evidence of that entity? If
the answer is yes, and yet no such evidence appears, then an AfS is
strong. If the answer is no, then it is weak.
But an AfS that gets this far can be made stronger if we can fulfill
either of two more criteria. First, is the hypothesized entity the sort
of thing, based on long experience with other examples of the same kind,
that is easily arrived at by the human imagination even when not real?
If the answer is yes, then an AfS gains strength. If no, then it
actually loses strength. Second, does the hypothesized entity entail or
include properties that we know on abundant, reliable evidence cannot or
do not exist? If yes, then an AfS gains strength. If no, then it
doesn't. Note that this second criterion does not rule out such claims.
Rather, it only strengthens a preexisting doubt. Enough evidence can
indeed confirm the seemingly impossible and prove it possible, but we
are not considering a case where the evidence is strong or abundant.