From ancient Greece, two traditions of rationalism have been bred in Western society. One is negative rationality, which contains two main connotations: （1）recognizing the shortcomings of human rationality; （2）recognizing the irrationality of the real world. Negative rationality leads to the thought trends of pluralism, evolutionism and empiricism, and then shapes a kind of contradictory, dialectical and critical two-dimensional thinking mode. The second is positive rationality, which contains two main connotations: （1）affirming human rationality; （2）affirming social reality. Positive rationality gives rise to the thought trends of monism, constructivism and transcendentalism, and then shapes a kind of one-dimensional thinking mode without negativity, criticism and transcendence. At the same time, because the thinking and cognition is from the exploration of nature, the rational thinking in Western society shows the trend of development from negation to affirmation, from evolution to construction. As a result, positive rational thinking is expanding and eventually gains a dominant position; accordingly, it lays the basic characteristics of thinking in Western society, and further constitutes the philosophical and methodological basis of modern economics.　　However, there are serious contradictions between the two main connotations of positive rationality. On the one hand, the affirmation of human rational ability brings out rationalism, which stimulates people to establish a perfect ideal society; on the other hand, the affirmation of the real world derives social Darwinism, which promotes people to accept reality without criticism. Applying this to social practice also brings great confusion: the former leads to " fatal conceit” in the sense of Hayek, and the latter leads to " one dimension state” in the sense of Marcuse. In fact, rooted in positive rational thinking, the modern mainstream economics tries to build a rational choice framework. On the one hand, it designs and guides social system reform based on the rational model; on the other hand, it rationalizes the reality based on ethical positivism. Furthermore, its social cognition and policy propositions tend to shift between two extremes: on the one hand, it turns to Keynesian economics with active intervention when the spontaneous market fails seriously; on the other hand, it turns to neoclassical economics with laissez-faire when government intervention fails seriously.　　Therefore, the theoretical system of the modern mainstream economics contains serious internal thinking tension and logical fault, which severely limits its ability to find and solve problems. Obviously, this inspires us to re-examine the positive rational thinking embedded in modern mainstream economics. In fact, human rationality is negative in essence, which embodies the ability to observe and discover the real existence. Unfortunately, positive rationality unifies thinking and reality, thus reflecting the alienation of human thinking.