Cathexis: def. The investment of energy in an object, idea, or person is known as cathexis. Since the id does not distinguish between a mental image and reality, it may not lead to direct action to satisfy a need. Instead, the id may simply form an image of the desired object that is satisfying in the short term but does not fulfill the need in the long term. For example, a person who is hungry may create a mental image of a desired food rather than actually eating.