Let us assume that there is a perfect antivirus program. It examines a program and says "Yes" if the program will infect our computer. It says "NO" if the program will not infect our computer. Antivirus (Program): If Program causes infection Report "Yes" Else Report "NO" If such a program existed, we could write a program Evil that accepts a program as its input and asks Antivirus to look at it. If Antivirus says "Yes", Evil stops without doing anything else. If Antivirus says "NO" about the program, Evil infects the computer: Evil (Program): If Antivirus (Program) says "Yes" Exit Else infect computer Make an argument for how this program Evil shows that the program Antivirus cannot be written