Developing critical thinking ability is a crucial purpose of education. Among critical scholars, British scientific philosopher, Harvey Siegel's work impressively demonstrates the relationship between rationality and education, and considers critical thinking ability as an end of education. This research, based on his perspective on epistemology, traces Siegel's theory concerning the connection between rationality and critical thinking, to reveal four reasons to take critical thinking as an educational purpose-respect for students as persons, self-sufficiency and preparation for adulthood, initiation into rational traditions, critical thinking and democratic living. The paper then applies the connection to a consideration of critical thinking and education.