Robert Pogue Harrison frames Forests with a quotation from 17th century philosopher Giambattista Vico:
This was the order of human institutions: first the forests, after that the huts, then the village, next the cities, and finally the academies. [The New Science §239]
Harrison’s core argument is that Vico has latched onto a fundamental truth about the development of human culture in this developmental history from forests to academies. Harrison reads a long history of European philosophical and literary texts in order to explain that the concept of a forest is fundamental to that historical tradition.
The opening chapter of Forests is dedicated to the difference between a forest and all other spaces. Drawing from Vico’s analysis, Harrison claims that the primeval forest is the location through and against which subjectivity is formed. The trees wholly envelope the land they take over, and it is the act of forming a clearing through which a species is able to gain access to logos, or in Harrison’s terms, a “horizon of sense.”
This close relationship between human modes of signification and the human relation to and distance from forests is a cornerstone of this work. Logos is formed as a byproduct of mastery over the forest as a structure, but the forest never quite leaves the species. Harrison makes a few stops throughout to perform exegesis of various words that have come down through Greek and Latin with woodland-based origins, with ultimate point being that while humans left the forest in order to attain reason, the forest never quite left our systems of reason themselves.
The flip side of this argument is presented when Harrison moves away from the mythological roots of our connections with forests and into the juridical sphere of the middle ages. While the primeval forest of myth or folk tale is a shadowy space where you can lose your way, become entrapped, and die — into the early middle ages, the forests of northern Europes “were still vast, stretching across the continent like domes of darkness and the indifference of time.” Around this time, and throughout the middle ages, these forests became more and more delimited. In yet another gesture toward the relationship between language and the mass of trees and underbrush that we call “forest,” Harrison notes that the word itself is not so much a descriptor for a physical space as it is a juridical term. While Harrison notes that the term has an “uncertain provenance,” he points to the likely constellation of foris (outside) and forestare (to keep out, to place off limits, to exclude).
While Harrison has moved from mythology to the documented history of Europe, his argument hasn’t changed significantly. The world of language is birthed from the human emergence from the forest, but this flows backward; the world of language gives rise to a juridical understanding of forests. He makes this very clear — “a ‘forest,’ then, was originally a juridical term for referring to land that had been put off limits by a royal decree.” More than this, “without this particularized legal bureaucracy, forests cannot exist.”
The tension between these two lines of thought opens up Forests as a book about more than forests. The tense line between discourse and the practice of naming with a politics (the heart of law) and the affordances of an object, what it obscures and what it makes available, are at the heart of every object of study. Harrison provides fruitful analysis, however, because of the nature of forests; or rather, because there is no Nature to forests. The practice of acting toward a forest is impossible without first conjuring up the very concept of a forest. There is no there there, just an ecosystem of interacting agents that are named juridically and without their knowing.
This gives insight into all assemblages of agents, and Harrison invokes a method that might be useful for all of us. When we pick an object, it might do to look at the mythological origins of that object alongside the naming practices that call it into political being. Or, more clearly, it is worth looking at where we think something comes from and where something really is coming from.
The third category for thinking the forest in Forests is through its very materiality. This doesn’t just mean its size and mass and how it is possible to become so entangled that you die inside of it, but rather “what are the effects of forests on the world?” Harrison, who is mostly concerned with literature, focuses specifically on the ways that forests have enabled certain expansionist practices, particularly for the Roman Empire. He writes, quite beautifully:
Forests became fleets, sinking to the bottom of the wine-dark sea. Trees became masts, drifting among the waves of Poseidon. The temple to Poseidon at Cape Sounion, overlooking the waterway that leads into and out of the bay of Piraeus, is an inspiring monument still today, but the barren mountain on which it stands, as well as the entire surrounding landscape, now drenched with that brilliant Hellenic light, shows no traces of the forests that once covered them. 
For the Romans, forests were the engine of expansion and war, which enabled them to find more places with more, and different forests. The effect of this was deforestation, and in the case of a huge stretch of northern Africa along the Mediterranean, the desertification of a once-fertile land.[56-57]
Forests are spaces that provide phenomenological experiences, which are then named and given a particular existence in the discursive field of humans, but yet still have a materiality that can be exhausted in particular ways. Harrison quotes David Attenborough’s historical account of the Roman Empire, claiming that “North Africa was producing half a million tons of grain every year and supplying the huge city of Rome, which had outstripped its own agricultural resources, with two-thirds of its wheat.” I bring this us both to point out that the problems of Empire then weirdly resemble the problems of Empire now, but also that the loss of that productive land can understandably be put down as one of the many reasons for the decline, and eventual dissolution, of the Romans.
Forests afford a process of learning to name, naming, and extracting. Harrison gives a beautiful example of how we can live in relation to an assemblage, believe that we come to know it, and then weaponize it to use against itself. Harrison draws compelling conclusions about how forests operate this way with humans as their interactors, but the question remains: what other assemblages have this kind of tight-knit closeness with the human?