Abstract Reasons behind why children think in different ways have been established in various theories. Jean Piaget advanced a greatly influential theory that reflected his prior studies in the fields of biology and genetic epistemology. It is a theory that has been contended by many others, including that of the information-processing approach to cognitive development. It will be shown where these theories compare and where they contrast, in conclusion explaining why Mary's children think differently. WORD COUNT - 1573 (Introduction, Body & Discussion) 1651 (Including Abstract) The cognitive abilities of children have long been an issue for developmental ps