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?