Elinor believes that it is within human nature to be cool, calm, and collected. While she has a warm, loving, and affectionate heart with strong feelings, she believes in the importance of tempering one's feelings and putting rational thought above emotion. She herself is described as being the "eldest daughter, whose advice was so effectual, [and who] possessed a strength of understanding, and coolness of judgement" that her mother turned to her for advice (Ch. 1). We also see her express her belief that it is within human nature to remain strong and selfless, despite any personal torment one is going through.
We first see Elinor embody her beliefs concerning human nature when she stoically accepts Lucy Steele's news of her secret engagement to Edward and bears it in silence. Feeling jealous of Elinor since she heard Edward speak of her so often, Lucy decides to confide in Elinor about their engagement. When Lucy shows Elinor all the evidence she can of their engagement, including a letter from Edward, Elinor is described as being "almost overcome--her heart sunk within her, and she could hardly stand," but she managed to fight off the "oppression of her feelings" and was able to keep walking and talking with Lucy (Ch. 24). Not only that, she manages to keep it a secret from the rest of her family and even wants to keep it a secret as she knows it will upset her family a great deal. However, it takes a great deal of effort on her part to keep her emotions in check and to maintain her rational side, showing us that she believes human nature capable of thinking reasonably and keeping emotions under control.
Later, when Marianne is suffering from her own broken heart, Elinor begs her to try and control her grief, saying:
Exert yourself, dear Marianne ... if you would not kill yourself and all who love you. Think of your mother; think of her misery while you suffer. (Ch. 29)
But Marianne refuses. Even though Elinor has her own broken heart, she must selflessly try and comfort Marianne.
Elinor's view of humanity's ability to think reasonably and remain controlled contrasts greatly with both Marianne's and their mother's view. Both "valued and cherished" wild, violent emotion.