What does "nor can I doubt but that" mean?

Question

The following excerpt comes from Mary Shelley's Frankenstein.

Prepare to hear of occurrences which are usually deemed marvellous. Were we among the tamer scenes of nature I might fear to encounter your unbelief, perhaps your ridicule; but many things will appear possible in these wild and mysterious regions which would provoke the laughter of those unacquainted with the ever-varied powers of nature; nor can I doubt but that my tale conveys in its series internal evidence of the truth of the events of which it is composed.

Here, what does "nor can I doubt but that" mean? Does the speaker doubt that his tale conveys in its series internal evidence of the truth of the events of which it is composed, or does he not doubt it?

Answer 1

According to the Macmillan Dictionary, the expression "no doubt but that" is

used for emphasizing that something is certain

The use of nor can rather that no does not change the meaning.

This sentence therefore means that he "does not doubt that....".

This expression is archaic, but the expression "can't help but..." is still in regular use.

Victoria can't help but smile at the remark. - The late Shift- Freelan Justice 2014

Answer 2

Including (optional) but in such constructions was never common, as that NGram link shows. But it's extremely "dated" today, so I suggest you simply ignore that word (it makes no difference whatsoever to the meaning).

...nor can I doubt that my tale conveys ... the truth

I think nor can I doubt [statement X] is really just a "linguistic flourish" here. We usually use it after an explicitly negated element, extending the negation to a second element (It's not X, nor is it Y) - but there is no preceding element to justify that in the exact context as cited, so it's not really justifiable on syntactic grounds by today's standards.

The construction I do not / cannot doubt [that] X [is true] is just a somewhat florid / circumlocutious / wordy / roundabout / dated way of saying I'm sure X is true. But I'd also add that although it's still perfectly natural to say I don't doubt that or I can't argue with that to express wholehearted agreement with some preceding assertion, it's extremely rare for anyone today to say anything like I can not [can't, cannot] doubt that.