At the end of Act 1, Scene 2, Edmund is revealing some of his villainous thoughts in a brief soliloquy. He is succeeding in driving a wedge between his father, the Duke of Gloucester, and his half-brother Edgar, who is the legitimate heir to Gloucester's title and estate. Edmund considers himself fortunate in having a credulous father--that is, a father who is easy to deceive. Edmund has forged a letter to make their father think that Edgar is plotting against his life in order to succeed him immediately.

Edmund does not consider Edgar "credulous" or "gullible," but he thinks Edgar is so noble that he naturally assumes other people have similar noble motives. Edmund contemptuously calls Edgar's integrity "foolish honesty." This attitude is also characteristic of Goneril and Regan, as it is of many people living today. They do not believe that honesty is the best policy but that selfishness is the best policy. Shakespeare is showing through these three characters that we should not be too trusting. Some people have integrity, and some people only pretend to have integrity. Even the most wicked people can pretend to be totally honest. 

Hypocrisy is the homage vice pays to virtue.
                      --Francois de La Rochefoucauld

Edmund is a consummate hypocrite. He is always scheming to advance himself by wicked practices, but always pretending to be the soul of honesty. He manages to advance so far in life that he seems on his way to becoming one of the rulers of England. Regan, whose husband was killed by a wound he received in Act 3, Scene 7, is in love with Edmund. She loves him because he is so much like herself--selfish, crafty and unscrupulous. She inherited half of her father King Lear's kingdom early in the play by lying to the credulous old king about how much she loved him. No doubt Edmund plans to get the whole kingdom by practicing his villainy against Albany, who is married to Lear's other daughter Goneril. But Edmund is thwarted at the end of the play when Edgar appears and challenges him to a "trial by combat" in which Edmund is mortally wounded. 

The word "credulous" means being too ready to believe. It seems likely that Shakespeare intended for his audience to understand that Gloucester was only easy for Edmund fool because he was growing old and perhaps succumbing to senile dementia. Many old people in our times are victimized by swindlers because they are easy to deceive. 

Another example in Shakespeare of a younger man who, like Edmund's half-brother Edgar, is easy to fool because of his noble nature is Brutus in Julius Caesar. Brutus' worst mistake is in trusting Marc Antony. As Shakespeare sees Brutus, he is easy to deceive because he is noble and assumes that everybody is like himself. Cassius also manipulates Brutus easily until Brutus finally realizes that Cassius is a totally different kind of person, and they have their violent quarrel in the famous tent scene in Act 4, Scene 2.