Why did the disciples think they were seeing a spirit when Jesus appeared after His resurrection?

Les explains that the disciples were not expecting resurrection at all. They had no idea Jesus was going to die, and after He died they had no idea He would rise from the dead; that truth was totally blanked out for them. Even after seeing Him once, they still struggled to accept it. So when Jesus suddenly stood in their midst and said, "Peace be unto you," they were terrified and affrighted and supposed they had seen a spirit. Jesus asked why they were troubled and why thoughts arose in their hearts, then invited them to behold and handle His hands and feet, proving He was Himself. He said a spirit does not have flesh and bone. He also showed them His hands and feet, and when they still believed not, though full of joy and wonder, He asked for food, and they gave Him broiled fish and a honeycomb, and He ate. Les points out this was with His glorified body, flesh and bone rather than flesh and blood. Jesus then reminded them that all things written in the law of Moses, the prophets, and the Psalms concerning Him must be fulfilled, and He opened their understanding that they might understand the Scriptures.