The Introduction provides the rationale for the writing of a history of early modern experimental philosophy and introduces the book’s major themes. It opens with a discussion of the meaning of the term ‘experimental philosophy’ and explains how it should be differentiated from contemporary x-phi and the historiographical category of empiricism. We claim that ‘experimental philosophy’ initially referred to a method for acquiring knowledge of nature that prioritises observation and experiment over theory, but it soon became the referent for the movement of experimental philosophers – as its practitioners called themselves – and for the actual knowledge acquired by this method. The Introduction then sets out some of the broader philosophical context in which experimental philosophy emerged, including the role of principles, the two-step approach to developing a science of nature, the experimental/speculative distinction, its employment of a form of natural history deriving from Francis Bacon, and a clutch of philosophical problems that impinged on this new approach to knowledge acquisition. These include the problems of how we get epistemic access to the essences of material things, how to articulate the precise relationship between experiment and observation on the one hand and theory on the other, and the roles of natural history and mathematics in experimental natural philosophy. The Introduction concludes with a summary of each chapter of the book.

It is familiar that, in Phaedo 95e ff., Plato argues that causation requires Forms and that Forms are causes. According to the standard and mainstream view, Plato’s argument for this view relies on the view that Forms are self-predicative (i.e., the Form of F is itself F) and that the cause transmits its character to the effect. The chapter demonstrates that Plato’s argument depends on neither of these views. It shows that what the argument relies on is the view that, first, Forms are essences (i.e,. the Form of F is what it is to be F), and, secondly, causation/explanation is uniform (i.e., same cause if, and only if, same effect).

The chapter considers why, and in what sense, Plato thinks that Forms are separate from sense-perceptible things. It argues that the notion of separation Plato operates with is not a purely modal notion, but rather, an essentialist notion: A is separate from B, if, and only if, What A is makes no reference to B, but what B is makes reference to A. It demonstrates that, for Plato, Forms are, in this sense, separate from sense-perceptible things, because what the primary Forms are, such as oneness or likeness, does not have to make reference to sense-perceptible things.

The chapter considers the relation, in Plato's Phaedo, between knowledge and enquiry. It argues that Plato's account of knowledge is not independent of his account of enquiry and that certain reatures of his account of knoweldge are based on his account of enquiry.

The chapter considers why Plato thinks that Forms are unitary and changeless. It argues that this follows from Plato's supposition that Forms are essences and that the definition of essences must conform to certain requirements.

The chapter considers why Plato thinks that Forms are distinct from sense-perceptible things. It focuses on Phaedo 74 and Republic 478-480. It argues against the standard view that, for Plato, sense-perceptible things are subject to co-presence of opposites. Plato's view is rather that sense-perceptible things, in so far as they are simply perceived by the senses, are subject to co-presence of opposites.

The chapter is dedicated to the Sun Analogy in Republic. It shows that, for Plato, mind and world are co-ordinate: neither is prior to the other; and this is because both mind and world are co-explained by a single ultimate principle, which is super-ordinate to both mind and world: the Idea of the Good.

The chapter considers why, in Republic VII. 523-525, Plato associates Forms with judgements about sense-perceptible things. It argues that he does so because he argues that judgements about sense-peceptible things, if they are to conform to the principle of non-contradiction, must use certain Forms, and especially the Form of oneness or unity, to individuate sense-perceptible things.

The chapter considers Plato's claim, in Parmenides 135, that thinking about things requires Forms. It locates Plato's argument for this claim in 132 and, especially, in the second part of the dialogue (142ff.). It shows that the second part of the dialogue is just as much about thinking as it is about being.

This is the Conclusion, and it shows that that upshot of this project is that Forms simply are essences, not things that have an essence.

The chapter argues that Plato, in the Hippias Major gives due consideration to the question whether, for some qualities F, such as beauty, it is possible to give an account of what F is by pointing to an example-and-exemplar. He takes seriously, and gives cogent reasons in defense of, an affirmative answer to this question in a manner comparable to Geach – although he argues that these reasons lead to inconsistency, if combined with the view that it is possible to make comparisons in regard to F among significantly different examples-and-exemplars of a thing that is F.

The significance of this project ought to be evident. First, it shows that Plato’s Forms simply are essences, not things that have an essence and that Plato’s theory of Forms is just that, a theory of essence. Secondly, it shows that, if we have reason to raise and take seriously the ti esti (‘What is it?’) question, then we have reason to take seriously Plato’s theory of essence, the theory of Forms, with the remarkable characteristics that Plato attributes to essences and Forms. thirdly, it presents a challenge to those philosophers today who think that essentialism is an optional item on the philosophical menu. Finally, it presents a challenge to the tendency, typical to the essentialists of today, to think that the only option is Aristotelian essentialism, and that Platonic essentialism, if it can be made out at all, is simply Aristotelian essentialism with the addition of certain Platonic commitments that are both unattractive and optional. The projects demonstrate that Plato’s essentialism is a well-argued, rigorous and coherent theory, and a viable competitor to Aristotle’s essentialism.

The chapter considers why Plato thinks that Forms cannot be perceived by the senses. It argues that this is because Forms, or essences, cannot be defined by example and exemplar. It shows that this does not mean that Forms/essences can be known only a priori.