The introduction of my Norton Critical Edition of S&S got me thinking about this idea. In the introduction, Claudia Johnson suggests that a main theme of the novel connected to the title is what kinds of evidence characters consult when they form judgements about other people in this story?

She writes "The novel is organized around...probelms of knowing and assenting that baffle Marianne and Elinor equally...Marianne believes the world and the people in it are transparent...By contrast, Elinor hesitates to equate hope and knowledge; she wants formal proof."

There is a lot of this already in this first section, I find. Not only Marianne but her mother too moves quickly to certainty about various people or situations, often with very little to go on. They rely on their feelings and treat them as proof.

A few examples:

In Ch. 3

Mrs. Dashwood had been informed by her husband of the solemn promise on the part of his son in their favour, which gave comfort to his last earthly reflections. She doubted the sincerity of this assurance no more than he had doubted it himself, …His attentive behaviour to herself and his sisters convinced her that their welfare was dear to him, and, for a long time, she firmly relied on the liberality of his intentions.

A bit later in Ch. 3, with very little knowledge of Edward:

[Mrs. Dashwood] speedily comprehended all[Edward's] merits; the persuasion of his regard for Elinor perhaps assisted her penetration; but she really felt assured of his worth:

No sooner did she perceive any symptom of love in his behaviour to Elinor, than she considered their serious attachment as certain, and looked forward to their marriage as rapidly approaching.

In Ch. 4

[Mrs. Dashwood's] resolution was formed as she read [Sir John's letter about the cottage]. (Ch. 5) She relied so undoubtingly on Sir John's description of the house, as to feel no curiosity to examine it herself till she entered it as her own.

Also in Ch. 4, we have Marianne jumping to conclusions. Here is one example:

Marianne was astonished to find how much the imagination of her mother and herself had outstripped the truth.

"And you really are not engaged to him!" said she. "Yet it certainly soon will happen.

Elinor does not jump to conclusions, but instead requires proof:

Also from Ch. 4

She felt that Edward stood very high in her opinion. She believed the regard to be mutual; but she required greater certainty of it to make Marianne's conviction of their attachment agreeable to her. She knew that what Marianne and her mother conjectured one moment, they believed the next -- that with them, to wish was to hope, and to hope was to expect...

I am by no means assured of his regard for me. There are moments when the extent of it seems doubtful; and till his sentiments are fully known, you cannot wonder at my wishing to avoid any encouragement of my own partiality, by believing or calling it more than it is. In my heart I feel little -- scarcely any doubt of his preference. But there are other points to be considered besides his inclination.

This is something I hadn't really noticed before--how often Elinor is weighing the evidence, either in her thoughts, or thinking out loud, and how often Marianne and her mother very quickly make up their minds with almost nothing to base their opinions on other than their feelings, and yet they are 'certain' and have no doubts they have come to the right conclusion.

Is this part of their brand of sensibility? I've never equated jumping to conclusions with sensibility before, but I suppose it's all part of the willingness to go with one's feelings.