Questions concerning Ijtihad and schools of law

• What does ijtihad mean?

• Is the door to ijtihad open?

• What are the factors which prevent the desires for ijtihad from being "heavenly" and make them "worldly"?

• Were all the companions free of lying?

• Why are there more than one school of law?

• The truth is one; How then can the different ordinances of four, or, as there were once, twelve schools be true?