In thermodynamic, metabolic, and other relationships, one system or unit gains or grows at the expense of the other: heat is transferred, one body gaining and the other losing it; food is metabolized with gain for the eater and chemical breakdown of the food. Cross-fertilization of ideas and knowledge between various sciences is advantageous but hazards arise unless one examines critically the concepts borrowed. In psychological interchange between parent and child, teacher and student, and analyst and analysand, both parties to the interchange can 'grow' and gain. In fact, most human interactions must benefit both parties to them if they are to benefit either. Szasz discusses the economy of exchange between the 'giver' and the 'taker'.