“I know not how it was – but, with the first glimpse of the building, a sense of insufferable gloom pervaded my spirit.”

The narrator describes what happens to him when he first sees the building: a feeling of overwhelming gloom comes over him. This is an experience he has in the story, not something he knows about the past. Although he says he does not know why he feels this way, the sentence reports the feeling itself rather than an inference about its cause.

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