Preliminary Discourse to the Encyclopedia of Diderot
Jean le Rond d'Alembert
 In part one of the book, d'Alembert provides a general introduction to the origin of knowledge, which led to the works found in the Encyclopédie. He asserts that the "existence of our senses" is "indisputable," and that these senses are thus the principle of all knowledge. He links this idea to a chain of thinking and reflection that eventually leads to the need to communicate, which sets another chain of events in effect. One of his arguments for the origin of communication is that it was necessary for people to protect themselves from the evils of the world and to benefit from each other's knowledge. This communication led to the exchange of ideas that enhanced the ability of individuals to further human knowledge. Additionally, d'Alembert introduces the reader to the types of knowledge people store. The two main types that he describes refer to direct and reflexive knowledge. Direct knowledge is obtained by human senses and reflexive knowledge is derived from direct knowledge. These two types of knowledge lead to the three main types of thinking and their corresponding divisions of human knowledge: memory, which corresponds with History; reflection or reason, which is the basis of Philosophy; and what d'Alembert refers to as "imagination," (50) or imitation of Nature, which produces Fine Arts. From these divisions spring smaller subdivisions such as physics, poetry, music and many others. d'Alembert was also greatly influenced by the Cartesian principle of simplicity. In this first part of the book, he describes how the reduction of the principles of a certain science gives them scope and makes them more "fertile" (22). Only by reducing principles can they be understood and related to each other. Ultimately, from a high "vantage point" (47) the philosopher can then view the vast labyrinth of sciences and the arts. d'Alembert then goes on to describe the tree of knowledge and the separation and simultaneous connections between memory, reason, and imagination. He later explains that the ideal universe would be one gigantic truth if one only knew how to view it as such; the assumption that knowledge has intrinsic unity can be seen as the foundation of the project of making the encyclopedia. Part two of the book provides the reader with an account of the progress of human knowledge in the sequence of memory, imagination and reason. This sequence is different from the one described in Part I, where the sequence is memory, reason, and imagination. It is the sequence a mind left in isolation or the original generation follows whereas in Part II he describes the progress of human knowledge in the centuries of enlightenment that started from erudition, continued with belles-lettres, and reached to philosophy. Instead of writing in terms of general ideas, d'Alembert provides the dates, places and people responsible for the progress of literary works since the Renaissance leading up to his date. One key example is René Descartes, who the author lauds as both an excellent philosopher and mathematician. His application of algebra to geometry, also known as the Cartesian coordinate system, provided an excellent tool for the physical sciences. He focuses on the importance of ancient knowledge and the ability to understand and build on it. Reference is made that concepts of knowledge could not have advanced as quickly had there not been ancient works to imitate and surpass. He also clarifies that there can be disadvantages to the ability to retrieve information from the past. Noted in the text is the lack of improvement in philosophy in comparison to other advancements due to the ignorant belief that ancient philosophy could not be questioned. d'Alembert claims that it would be ignorant to perceive that everything could be known about a particular subject. Additionally, he makes an attempt to show how individuals could free their minds from the yoke of authority. His use of deductive logic provides a more philosophical base for the existence of God. He makes clear that all sciences are restricted as much as can be to facts and that opinion influences science as little as possible. d'Alembert states that philosophy is far more effective at the analysis of our perceptions when the "soul is in a state of tranquility", when it is not caught up in passion and emotion (96). He believes that the philosopher is key in furthering the fields of science. The philosopher must be able to stand back and observe science and nature with an impartial eye. Furthermore, the importance of science and the advancements of such intellects as Francis Bacon, Isaac Newton, Descartes, John Locke and others are explained. Part three of the book concludes by detailing the important attributes of the writing of the Encyclopédie and by mentioning important contributors. d'Alembert discusses how the Encyclopédie is open to changes and additions from others since it is a work of many centuries. In addition, he states that an omission in an encyclopedia is harmful to its substance which differs from an omission in a dictionary. d'Alembert also states the three categories of the Encyclopédie, which are the sciences, the liberal arts and the mechanical arts. He states that it is important that these subcategories remain separate and concludes with the fact that society must judge the Preliminary Discourse to the Encyclopedia of Diderot. At the end of the book, d'Alembert includes a detailed explanation of the system of human knowledge. This includes a chart entitled "Figurative System of Human Knowledge", which divides human understanding into its three constituents: memory, reason, and imagination. The chart then subdivides each of the three major categories into many other categories of human understanding. After his chart d'Alembert goes on to provide a detailed explanation of every division and subdivision apparent in his chart. The chart establishes a complex genealogy of knowledge and the way man has subdivided knowledge into the specific areas he feels they are applicable. It is important to remember that no one of these systems of human knowledge plays a more significant role than any of the others. These systems are designed around the idea that each uses the other two to build upon itself and further human knowledge as a whole. In context the chart shows a progression of knowledge through the ages, memory being the past, reason being the present, which examines and tries to either build or create new theories based on memory, and imagination which focuses on making new assumptions or theories about things in our human universe.