The ideology behind the suspension of disbelief was coined by Samuel Taylor Coleridge in 1817. This idea has been applied to texts such as Mary Shelley's Frankenstein and Coleridge's own "The Rime of the Ancient Mariner." This concept is defined through the idea that if a reader can find a small aspect of truth in a text, he or she would suspend judgment on the things which seem utterly implausible.
Given that much of Greek/Latin mythology, or mythology in any culture, defines reasoning behind why something is as it is (in regards to origin or creation), one must suspend his or her judgment whether or not something could actually happen or not. One must simply choose to accept the myth as acceptable based upon a singular truth. In the case of Echo and Narcissus, one must choose to believe in the tale of the two based upon their understanding of the echo. For many, their knowledge of the echo helps to allow them to rationalize the myth as containing a singular truth and, therefore, be believable (at least for the moment).
At the same time, readers' prior knowledge regarding how water acts when it is touched allows them to find an instant of truth in the story of Narcissus. Here is where the singular truth lies in this aspect of the text. Again, readers must only find one aspect of the text to grasp onto in order to suspend their judgment on the rest of the text. Since water loses it reflective nature when touched, one could see this as a reality in the myth. Therefore, the reader will suspend his or her disbelief regarding Narcissus turning into a flower.