Although logical subject matter is easier to individualize into programmed materials, researchers have not found any subject matter that could not be programmed. During his 60-year career, Skinner discovered important principles of operant conditioning, a type of learning that involves rewards and punishments. A strict behaviorist, Skinner believed that operant conditioning could explain even the most complex of human behaviors. Individualized instruction was originally presented in book form, and sometimes still is. In order to prevent students from looking at the answers in the book ahead of time, Program Instruction became automated by inserting it into a teaching machine.
Teaching machines are devices that house, display, and present printed programmed instruction.
Feedback is given when the program is advanced through actuation of a lever and the correct answer comes to view. Programmed instruction is linear, in that the author of the materials decided what step to present next, and that step is presented, no matter what the learner wants. In this way students could skip steps they already knew, or study remedial material on information already presented.