रज्जु means rope and सर्प means serpant. Rope in the dark give an impression of a serpant (serpant in the dark) and a gives an impression of a rope. This "Nyaya is conceptualised on the basis of this.
There is much difference between the true and unture fact but darkness may lessen the difference. For example, rope is rope and a sepaint is a serpant. They are not at all similar. Rope is inert and a serpant is live. That what is inert cannot move but that what is living can shake, swing and can move. Thus there is much difference between rope and a serpant. A person recognises the easily in light but when it is dark it becomes difficult for a person to differentiate between the two. In such a situation a man may get an impression of serpant in a rope and of rope in a serpant consequently, he does not fears where he ought to have fearted and fears when he need not to see that such a situation does not arise he must take recourse to light.
A person develops (infuses) an imagination, an idea about the other person according to his own thinking/belief. And then he behaves with that person accordingly. In such a situation sometimes a person suffers unnecessarily. In order to see that such a situation does not arise he must, with all efforts try to come out of the darkness of ignorance. He must always behave accepting the reality.