Tom Cornsweet’s Seeing, How Light Tells Us about the World was published in the year that he died, and it generously shares with us much of what he devoted his professional life to as a scientist concerned with human vision. Although the book’s title suggests that it is about seeing and light, in fact it also tells us something about the world. For example the book opens with a firmly naturalist assertion that ‘[t]he Earth was formed four billion years ago”(p.1), but almost as quickly the human is decentered from this idea of the Earth as he affirms that  ‘…our senses provide us with an astonishingly small fraction of the information that we are actually embedded in and, and we have generated our conceptions of the physical world on the basis of the extremely limited range of things in the physical world that can be detected by our physiology’ (p.1). Rather like David Hume, he suggests that most of what we claim to be truths about the world, for example the idea of space as empty, is simply a reflection of our inability to sense very much, and in the case of the eyes their limited response to a very small band in the electromagnetic spectrum. To give some tangible scale to this restriction, he suggests that if the distance between AM radio and X-Rays on the electromagnetic spectrum was represented by the distance between New York and Los Angeles, then the human visible bandwidth would be less than an eighth of an inch. Once that is clear the majority of the book is concerned with explaining with patience and care how the human eye works – at least as far as we understand it with our limited sensibility amplified - or not (depending on where you stand in the objectivity debate) - by instruments.

The book opens with some basic, and possibly familiar. Explanations of light and a careful description of the anatomy of the eye, but where the real work begins is in Chapter Three in which he describes how photoreceptors sense light. It is here that the familiar classroom models of lenses, rods, cones and pin hole cameras, linear perspective, and so forth dissolve as he turns our attention to  ‘… the visual phenomena … [that] depend, initially on the events and processes that occur when light interacts with the light detectors in our eyes.’ In short; a few photons strike one of the billions of pigment molecules in the retina and the additional energy changes its shape (bleaching it) so that it disrupts the flow of electrical energy across the retina. The molecule remains in this ‘bleached’ state for anything between one and seven minutes. This happens on a very large scale and how particular molecules are changed is a matter of chance, but probability theory provides reliable certainty that when we move from a gloomy space to a light one the image retains its contrast.  Cornsweet proposes in establishing this interaction between the world and the eye at a molecular level is to avoid devolving difficult questions about visual perception to the black box of the brain and to account for them as a retinal function. For him much of what happens is an aggregation of simple interactions between a vast number of pigment molecules each changed by the impact of a very small number of photons. Most significantly, it is a process that is affected by energy sources in the external world as well as independent events within the retina so that we can ‘see’ things that are not ‘there’.

The phenomena of vison without external stimuli is well recorded and discussed, as various philosophers have puzzled about what happens when we press on our eyes or stare at bright lights and look away, etc. Cornsweet also devotes a chapter to what happens when we shut our eyes. However, the visual noise, flashes and stars, etc. that are caused by spontaneous bleaching of pigment molecules in his description of the process are of a different order since these are part of the function of the eye and are indistinguishable from external visual stimuli. This approach to understanding vision as a molecular change subject to random events governed by probability provides an alternative account of how we see, while it simultaneously proposes that there is a continuity between the operations of perception and those of the external world that suggest our claims for naturalism must be both modest and contingent.

Cornsweet’s career as a scientist has been devoted to human vision and developing instruments that allowed him to understand part of its workings. His name is given to a particular optical illusion that reveals some aspects of how we privilege contrast over tone in visual perception independently of the data that is presented. In the Cornsweet illusion tonal differences are perceived in a flat plane if a small darker band is placed across a field of uniform luminance. Many illusions with similar effects are often used to show the superiority of science and reason and how the eyes are unreliable sensors that fool the brain into misunderstanding the truth of the world. This is not Cornsweet’s object; instead, he is concerned with explaining the function of the eye that leads to this perception and, as such, for him it is not so much a question of the optical illusion fooling the brain as the way in which the world and the eye collaborate to reconcile quite different versions of the external world that are equally true.

Seeing, How LightTtells Us about the World is a paradigm of late style that reflects on Cornsweet’s intellectual capital with a secure confidence that leads to a simplicity and clarity that belies the complexity of his insight. He does not say as much, but the intellectual generosity of his style – his refusal to complicate and mystify, for example by using simple illustrative numbers, bear the mark of a scholar at peace with the limitations of what is known. Indeed at the end of each chapter he invites the reader to think about some of the things he has just been explaining in ways that inevitably expose the limits of his explanation. Cornsweet died in November 2017, and in an obituary, someone remarked that sooner or later, everyone who met him became his student. This phrase describes exactly what the book does and how it engages the reader as it deals with the simple and the complex as contingencies to be encountered.