The University of Edinburgh's Pauline Phemister is a lecturer in the history of modern philosophy with an emphasis on Leibniz. Her present work on the three most influential rationalist thinkers in the history of philosophy is an excellent intermediary level excursion for those who are familiar with Descartes, Spinoza, and Leibniz in only an introductory way. Phemister's monograph further offers a helpful comparison of three distinct systems of philosophy, which any philosopher knowledgeable of these thinkers would find insightful.

In citing a particularly appealing methodological aspect of the philosophy of Leibniz, Phemister seems to also lay out her approach to the subject of her book, “Leibniz's approach in the furtherance of knowledge was to seek out agreement and minimize disagreement. Commenting on the views of others, he generally eschews outright refutation, preferring to concentrate on points of agreement that each party could regard as true” (p. 16). Likewise, Phemister does not take these philosophers to task over every minute detail within their respective systems. As such, her approach to the subject is to find the maximal amount of agreement between the three philosophers in question without omitting obvious areas of disagreement. Such a charitable approach to the history of philosophy is both laudable and helpful.

In her introduction, Phemister begins by offering a concise biographical sketch of each of the philosophers, wherein their interaction of thought is clearly delineated. Spinoza and Leibniz both were well versed in Cartesian thought. Both held onto some aspects of Descartes' philosophy while eliminating others from their own systems. All three were assisted in their development by correspondences with academic peers whose questioning helped refine their respective systems. Phemister also outlines the rest of the book at this point. She begins with the total systems of each philosopher, moves on to epistemological questions, follows up with metaphysical considerations, and finishes with ethical implications.

In Chapter one Phemister argues that each of the philosophers had a similar goal in mind. Each sought to build a unified system of philosophy which culminated with ethical theory. However, each took a different approach as to how to build this unified system. In comparing the three, Phemister chooses to begin with an analysis of Descartes' methodology. She follows this up by comparing and contrasting Spinoza and Leibniz with the method that Descartes delineated. Some significant divergence is found between the three.

Chapter two deals with the ways the three thinkers approach epistemology. Knowledge and ideas are the two main components that are explicated at this point. Phemister expertly analyzes first the question of how we know what we know. At this point the focus shifts to Leibniz as an intermediary between Descartes and Spinoza. Leibniz seems to find a middle ground. Leibniz agrees with Descartes on the need for clear and distinct ideas to formulate knowledge, while disagreeing with him on the extent to which our knowledge can reach. In disagreeing with the Cartesian methodological doubt, Leibniz ends up with more sure knowledge of the external world. This is an area of agreement between Leibniz and Spinoza, though the two disagree over how to attain such knowledge. A corollary issue is that of the ontological status of ideas. It is here that Spinoza takes a path that leads into questions of metaphysics. Spinoza's notion of ideas as modes of a mind finds some agreement with Descartes, and offers Phemister a natural transition into metaphysics–beginning with questions of substance.

Chapters three through nine all deal with some aspect of metaphysics. Chapter three is an investigation into how each of these rationalists define substance. Here we encounter the monad of Leibniz. In seeking to avoid the implications associated with Cartesian methodological doubt, Leibniz formulates a metaphysic wherein knowledge is affirmed by the postulation of each monad containing within itself a reflection of every other monad. Both Descartes and Leibniz allow for substance to include finite things, while Spinoza does not. The bulk of the effort in this chapter is spent comparing Descartes and Leibniz, who have much more in common with one another on the subject than with Spinoza.

Chapter four outlines Spinoza's formulation of metaphysics; which is impossible to consider without going into detail about his understanding of God and nature as a singular being–recognizable categorically as thought and extension. This monistic understanding is worked out in every area of Spinoza's philosophy. This disallows for substance to include finite things. For Spinoza there actually are no finite things, only necessary modes of the infinite. Phemister offers an erudite analysis of Spinoza's God, raising questions that Spinoza's system has difficulty in answering.

Together, chapters three and four give a broad understanding of the metaphysics of each thinker. This broad understanding sets the stage for a closer look at issues such as mind and body. Phemister continues on such metaphysical topics through the end of chapter nine. In those chapters, she argues that Leibniz (in a number of ways) ends up appropriating the best of both Descartes and Spinoza, while avoiding some of their pitfalls.

The final two chapters deal with questions pertinent to ethics. The ethical implications to be discussed are logically dependent on both the epistemology and the metaphysics of the individual philosophers. Specifically, the question of freedom (of both God and any possible contingent beings) and resulting responsibility is explored as related to each cohesive system. Once again there is similarity and divergence of thought brought into light. One interesting sidebar Phemister addresses is the question of possible worlds. Leibniz formulates a resolution to the problem of evil based on possible worlds, while Spinoza is unable to admit any possible world other than this one. Concomitantly, for Spinoza, God has no freedom to do anything other than what is necessary.

Phemister clearly contrasts today's philosophers (who tend to specialize in either an analytic subcategory of philosophy or on a particular philosopher) with these prolific rationalists who attempted to develop coherent and comprehensive systems of philosophical thought. In attempting to compare and contrast these thinkers, Phemister has avoided needlessly critiquing each point along the way. However, she does not avoid portraying strengths or weaknesses in any of the three systems. The outcome of Phemister's total effort is an exceedingly well outlined and argued analysis of these preeminent rationalist philosophers. I commend this volume to anyone seeking a better understanding of the interrelated ideas of these philosophers or how their epistemological methods logically ended in diverse metaphysical systems.