Popper saw problems with Induction that the logical positivists grounded their work on. The problems with induction is that you cannot be certain of something. We have always believed science to be true but it is based on induction.
Although the Logical Positivits say we can prove if a statment is true using validitilty Popper attacks this and says although a statment can be verfiied now it can still be proved wrong. Everything is open to falsification.