There are individual differences in rational thinking that are less than perfectly correlated with individual differences in intelligence because intelligence and rationality occupy different conceptual locations in models of cognition. A tripartite extension of currently popular dual-process theories is presented in this chapter that illustrates how intelligence and rationality are theoretically separate concepts. Thus, individual differences in the cognitive skills that underlie rational thinking must be studied in their own right because intelligence tests do not explicitly assess rational thinking. We close the chapter by describing our attempt to develop the first prototype of a comprehensive test of rational thought, the Comprehensive Assessment of Rational Thinking (CART). With the CART, we aim to draw more attention to the skills of rational thought by measuring them systematically and by examining the correlates of individual differences in these cognitive skills.