It is a brainstorm apropos of nothing. Ostensibly, it follows up the musings of the novel’s protagonist, Fanny Price, on changing landscapes, but is really a non sequitur, just a profound perception – pretty much identical to Freud’s – too exciting not to get down on paper.
Here it is, with its surrounding context.
“Three years ago this was nothing but a rough hedgerow along the upper side of the field, never thought of as anything, or capable of becoming anything; and now it is converted into a walk, and it would be difficult to say whether most valuable as a convenience or an ornament; and, perhaps, in another three years we may be forgetting – almost forgetting what it was before. How wonderful, how very wonderful the operations of time, and the changes of the human mind!” And following the latter train of thought, she soon afterwards added: “If any one faculty of our nature may be called more wonderful than the rest, I do think it is memory. There seems something more speakingly incomprehensible in the powers, the failures, the inequalities of memory, than in any other of our intelligences. The memory is sometimes so retentive, so serviceable, so obedient; at other, so bewildered and so weak; and at other again, so tyrannic, so beyond control! We are, to be sure, a miracle every way – but our powers of recollecting and of forgetting do seem peculiarly past finding out.”
Miss Crawford, untouched and inattentive, had nothing to say; and Fanny, perceiving it, brought back her own mind to what she thought must interest.
“It may seem impertinent in me to praise, but I must admire the taste Mrs. Grant has shown in all this.”
Of course, with the self-confidence of a great artist living in a stable society, Austen did not feel compelled to construct a cockamamie theory to explain the mystery.