Here, Jane introduces her superstitious...
Jane Eyre explains her thoughts of presentiments, sympathies, and signs at the start of chapter 20:
Presentiments are strange things! and so are sympathies; and so are signs; and the three combined make one mystery to which humanity has not yet found the key.
Here, Jane introduces her superstitious beliefs. After, she elaborates on each of these three words.
I never laughed at presentiments in my life, because I have had strange ones of my own. (chapter 20)
Presentiments are predictions or premonitions about the future. Jane believes in presentiments because she has experienced moments where her gut feelings predicted future events.
Sympathies, I believe, exist (for instance, between far-distant, long-absent, wholly estranged relatives asserting, notwithstanding their alienation, the unity of the source to which each traces his origin) whose workings baffle mortal comprehension.
Sympathies, she explains, are common understandings or emotions between different individuals. There is an inexplicable connection (or sense of understanding) between some individuals, despite the distance between them:
And signs, for aught we know, may be but the sympathies of Nature with man.
By signs, Jane refers to events or objects that signal or predict a future event.
Jane's superstitious beliefs are shown throughout the novel. Some examples of signs, sympathies, and presentiments include the following:
1.) Early in chapter 20, Jane explains that dreams of children are a sign of trouble to come:
I one night heard Bessie Leaven say to Martha Abbot that she had been dreaming about a little child; and that to dream of children was a sure sign of trouble, either to one’s self or one’s kin. (chapter 20)
This dream is particularly concerning to Jane because she's been dreaming of children lately:
For during the past week scarcely a night had gone over my couch that had not brought with it a dream of an infant. (chapter 20)
2.) An example of sympathies is seen in chapter 22 when Jane explains how Mr. Rochester seems to understand her thoughts and feelings without Jane explaining them:
Mr. Rochester had sometimes read my unspoken thoughts with an acumen to me incomprehensible." (chapter 22)
These are two of many examples of presentiments, sympathies, and signs in Jane Eyre.