Spinoza doesn't define 'essence' in Part I of the Ethics because he takes the meaning of that term to be well-understood: the essence of a thing is its nature. (Descartes, whose work Spinoza knew quite well, uses the terms 'essence' and 'nature' interchangeably in the Principles of Philosophy , Part I, Article 53.) A self-caused being, or causa sui --of which there is only one instance, God--is such that its very nature requires that it exist. Spinoza says a bit more about the term 'essence' in the second definition of Part II. "I say that to the essence of any thing belongs that which, being given, the thing is necessarily posited and which, being taken away, the thing is necessarily taken away; or that without which the thing can neither be nor be conceived, and which can neither be nor be conceived without the thing." The first axiom of Part II then distinguishes human beings from a causa sui : "the essence of man does not involve necessary existence."

Spinoza's Ethics is an extraordinarily difficult work. I find that it is one of the two most difficult texts written by an early modern philosopher: the other is Hume's Treatise of Human Nature . One reason for the difficulty of Spinoza's text is its style: Spinoza's geometrical method is designed to preclude the reader from attending to anything but the particular propositions of the work, and their proofs, and consequently, it is most difficult for the reader to find her bearings in the work. But Spinoza was not writing in a vacuum. He had been steeped in Descartes' writings, and even wrote a geometrical presentation of Descartes' Principles of Philosophy , the Principles of Cartesian Philosophy . Many of the claims of the Ethics implicitly engage Descartes. Consequently, I have found that it is helpful to read Descartes' Meditations , Part I of his Principles of Philosophy , and the Passions of the Soul , in order to have a point of reference for Spinoza's claims about...

One way to understand the structure of Kant's arguments for the existence of pure forms of intuition (space and time) and the categories of the understanding (such as causality), is to see them as starting from the premise that we have knowledge of a certain sort (for example, knowledge of geometry or arithmetic, or knowledge that things change), and showing that in order for us to have this knowledge, the cognitive faculties must be structured in a certain way. With respect to causality, Kant's claim is that the principle of cause and effect underlies our experience of things as causally related and therefore cannot be derived from that experience, but instead makes it possible. Kant presents the argument for this claim--which is considerably more complicated than I've represented it--in the Second Analogy of experience in the first Critique .

In the first line of the Introduction to the B (second) edition of the first Critique , Kant says that "there is no doubt whatsoever that all our cognition begins with experience, for how else should the cognitive faculty be awakened into exercise" (B1). So experience is necessary in order for human cognitive faculties to operate, and "no cognition in us precedes experience, and with experience, every cognition begins" (B1). Kant goes on to claim in the next paragraph that although knowledge (cognition) begins from experience, there may be some non-experiential component to knowledge provided by the cognitive faculty, "merely prompted by sensible impressions" (B1). He's not assuming that there is such a non-experiential component to knowledge, but simply asking whether there is any such non-experiential component to experience. This is, arguably, the guiding question of the first Critique .

According to Leibniz, mere material things--like food--are not monads. So Leibniz doesn't believe that human bodies are monads, either. But this doesn't dissolve your question. For the fact remains that human beings have experiences as of eating food, and experiences of their bodies, and experiences of their bodies as being nourished by eating food, and so you might well wonder how Leibniz would explain that experience, or any phenomenal experience, for that matter. In order to answer this question, one has to draw a distinction between the phenomenal and metaphysical levels of analysis. At the metaphysical level, according to Leibniz, all there are are monads, which consist of perception and appetition (perception and desires). Leibniz's metaphysics explains all our experience in terms of these perceptions and desires. So, according to Leibniz, alll our ordinary experiences--as of eating food--have their metaphysical basis in the nature of the monad. However, this metaphysical claim does not...

Just a couple of remarks about Descartes. First of all, Descartes doesn't even use the phrase, "I think, therefore I am" in the Meditations ; the phrase only appears in the Discourse on Method . In the Meditations , Descartes writes: "So after considering everything very thoroughly I must finally conclude that this proposition, 'I am, I exist', is necessarily true whenever it is put forward by me or conceived in my mind." What this 'I' is, however, is another matter, which Descartes goes on to examine further in the Second Meditation. In fact, in the Sixth Meditation, Descartes purports to show that the 'I' is a thinking thing. (This raises a vexing question: what does Descartes take the essence of thought to be?) Although 'I am, I exist' is the first truth discovered in the Meditations , it's not the only truth: in the Third Meditation, Descartes purports to discover that God exists; in the Fifth Meditation, he purports to discover the essence of body is extension; in the Sixth...

Peter Lipton's reading of Hume is an instance of the skeptical reading of Hume that has prevailed since the eighteenth century. There is, however, another way to read Hume, that has become especially widespread in the last ten years or so, but which also has a long pedigree. On this reading, the naturalist reading of Hume, Hume's question about causal reasoning, the inference from the observed to the unobserved, from impressions to ideas, is a question about which cognitive faculty (reason, the senses, or the imagination) enables agents to draw this inference. Hume's conclusion is that this inference is based on the imagination. (In this way, Hume can be seen as seeking to satisfy his "curiousity...to learn the foundation of this inference.") But this conclusion need not be taken to imply that Hume does not believe that causal inferences cannot be justified or unjustified; indeed, Hume articulates 'rules for judging of causes and effects', and in certain places, seems to suggest that there are norms...

In recent years, there has been an upsurge in interest among Anglo-American philosophers in such philosophers such as Simone de Beauvoir, Maurice Merleau-Ponty, and Jean-Paul Sartre. In a recent book, Authority and Estrangement: An Essay on Self-Knowledge , Richard Moran draws on Sartre's Being and Nothingness in order to give an account of first-person authority. I think that there is much in Being and Nothingness that could illuminate such questions as the nature of human freedom and the nature of our knowledge of other minds. Sartre's writings deserve further consideration from Anglo-American philosophers.

In Sections 110-117 of the Principles of Human Knowledge , Berkeley takes up questions in natural philosophy. He argues that space cannot exist without the mind, any more than other objects can. He also discusses space in An Essay Towards a New Theory of Vision and in De Motu .

There are newer translations of many of Nietzsche's works in the series, Cambridge Texts in the History of Philosophy . Hackett has also published fine translations of The Genealogy of Morals and Twilight of the Idols and may well be planning to publish further translations.