This book compares two important theories in recent times: the mental logic theory and the theory of mental models. While both theories acknowledge the role that pragmatics plays in cognition and language, there is an essential difference between them. The mental logic theory gives primacy to syntax. On the contrary, the theory of mental models considers semantics to be the basic factor. The book offers a description of these two theories, including their general frameworks and their ways to explain mistakes in reasoning activities. Thus, resorting to the specialized literature, it is shown that the theory of mental models has a greater explicative potential. The reason is that this last theory seems to be able to account for some experimental results that the mental logic theory cannot deal with. This can lead to assume the theory of mental models and reject the mental logic theory. However, it leads to a new problem as well: if the theory of mental models is mainly based on semantics, a key element such as syntax can be relegated to the background. Following author’s previous works, the book reminds that the theory of mental models can be compatible with syntax too. Hence, the problem may not be real.