I feel that the most important Foster motif which sustains itself throughout every era of literature is symbolism. A symbol could be manifested in many ways, be it objects, images or even actions, without any one definite meaning unless it is an allegory.
Foster indicates that the most important parts of decoding a symbol is by first considering allusions present in the text, considering the authors meaning of the use of the item, consider any ironic elements, and reaching a conclusion based on these things.
Symbols are prevalent in romanticism, with The Scarlet Letter being a prime example, where the symbol of the red letter ‘A’ on Hester’s chest symbolizes her sin and adultery, letter evolving to reflect her acts of charity and grace.
Symbols are also seen in realism, with the river in The Adventure of Huckleberry Finn.
The river symbolizes both danger and safety at different times, and an escape to be reborn, in order to have a fresh start.
Lastly, in modernism, we can see that the green light in The Great Gatsby symbolizes the emotional as well as physical rift between Gatsby and Daisy, the gap between Gatsby’s past and present as well as a hope for his future.