Sometime in the 1660s, John Locke and a handful of friends found themselves discussing morality and religion, but they soon reached an impasse, running up against "difficulties that rose on every side." From this common enough situation sprang the insight that gave rise to the Essay Concerning Human Understanding, a formidable achievement that would become the basis of modern empiricism. Why is it that such arguments so often end in stalemate? Locke saw that instead of giving our imagination free rein, we need to step back and determine the limits of human knowledge. He gave philosophy a critical turn by shifting attention from the subject matter of many debates - the nature of God, the ultimate truths of morality, the nature of the world itself - to the tools and materials of knowledge. If we are simply not in a position to discover these ultimate truths, we can free ourselves from useless wrangling and focus on what is within our grasp. What is more, we can see dogmatic pronouncements about matters that lie beyond human understanding for what they are: at best, nonsense; at worst, fig leaves to hide political or religious agendas. Since Locke hoped to have an effect on the public debate of his age, he wrote with considerable clarity on a wide range of difficult issues. His Essay Concerning Human Understanding is thus one of those rare books that is of as much interest to the intelligent reader as to the specialist.

The son of a Protestant landowner and part-time attorney, John Locke was born in 1632 in Somerset, England. Through his father's influence, he was able to secure a place at the prestigious Westminster School at the age of fifteen,ultimately going on to study at Christ Church, Oxford. Locke seems not to have formed any definite plans for his future after graduation. He took the first steps on the path to becoming a lawyer by enrolling in Gray's Inn. In 1658, he earned an M.A. at Oxford, the usual preparation for taking up a position in the Church, which seems to have been his father's wish. During this time he also studied medicine; this was to prove providential when in 1666 he met Lord Ashley (who would become the first Earl of Shaftesbury). After overseeing a successful operation on Ashley's liver (no mean feat in those days), Locke became his medical advisor and enjoyed his patronage. Shaftesbury vigorously opposed the succession of the heirless Charles II's Roman Catholic brother James. As part of Shaftesbury's circle, Locke was himself under royal suspicion. When in 1683 some of Shaftesbury's intimates were arrested in connection with the Rye House plot to kidnap Charles and James, Locke fled to the Netherlands, remaining there for six years, until the Glorious Revolution removed James II. From 1692 until his death twelve years later, Locke resided with Sir Francis and Lady Masham. He never married nor had children.

Locke's Essay Concerning Human Understanding was published in 1689 after his return to England, some twenty years after the conversation that was its impetus. If the epistemic modesty Locke enjoins seems a small reward for the effort of reading such an imposing volume, we should remember that dogmatism too often stands in the way of what little we can know about the world. Locke sees himself as an "under-laborer" in the service of science, sweeping aside the rubbish of other philosophers. Along with Francis Bacon, Thomas Hobbes, and a host of others, Locke did so thorough a job of cleaning house that we may find it hard to believe there was any debris there in the first place. But to complain that many of Locke's guiding ideas are truisms is like criticizing Hamlet for being full of clichés.

At the same time, much in the Essay remains controversial. The restriction of thought to ideas given in experience cuts the ground from under innumerable views in metaphysics and philosophy of religion. And the dictum that all meaningful speech is tied to ideas in the mind of the speaker, the chief theme of Book III, is indeed a demanding one. How many of us can claim to know what we're talking about when we hold forth on the religious and moral questions that gave rise to the Essay?

Although the Essay was almost immediately recognized as a major work, running to four editions before Locke's death, it was immediately controversial as well. In 1701, Oxford banned the Essay, expressly forbidding its professors to use it as a text.

What made the Essay so troubling to Locke's contemporaries? Western culture for centuries had been founded on what most took to be a single, unified tradition. In the work of St. Thomas Aquinas, for example, we find a clear expression of the intuition that the ideas of such disparate thinkers as Aristotle, Plato, and Augustine can be fit into a comprehensive worldview, with Christianity at its core. By the seventeenth century, however, it was no longer possible to pretend that this tradition formed a coherent whole. As the events of Locke's own life demonstrate, religious strife was pervasive and questions of theology were not recondite matters to be settled by learned theologians, but pressing questions of practical and political import.

Religious controversy bled into debate about the nature of the physical world. We must keep in mind that in this period there was no sharp distinction between science and philosophy. Descartes had quite as much to say on geometry, optics, and physiology as on metaphysics and epistemology. That such a distinction ever arose is partly due to Locke, since he is arguably the first to clearly articulate philosophy's critical role as an adjunct to science.

The Aristotelian natural philosophy that had dominated the Schools was slowly being supplanted by the new 'mechanical' philosophy, which took its cue from the atomism of Epicurus. For the Scholastics, the world is composed of substances, ordinary physical objects like horses, trees, and humans. Each naturally occurring substance is composed of a form or essence, which both makes the thing what it is and explains its causal powers, and of matter, the raw material that makes it up. A human being, in this view, is a form-matter compound like anything else in the sublunary world. The soul is the form of the person, but in Aristotle's own view, the soul, like any other form, cannot exist without matter. The Scholastics ran into difficulties trying to graft this view on to Christianity, and Aquinas spends a good deal of time trying to explain how the soul can outlive the body. The soul is part of a substance, the whole human being, and is not itself a substance or a 'this.'

By contrast, the mechanical philosophy sees the physical world as composed of atoms or 'corpuscles' that interact only by means of the mechanical forces of push and pull. There is no need for a form or essence of each thing. 'Horseness' and 'treeness' are nothing more than particular arrangements of microphysical particles. The most powerful proponent of the new philosophy was Descartes, who saw his job as providing the intellectual foundation of science. In his view, the tools for understanding the world in all of its richness are given to us by God in the form of innate ideas. The physical world can be fully understood in quantitative terms; anything irreducibly qualitative, such as thought and sensation, belongs to the mind or soul. And unlike the Scholastics, Descartes holds that the mind is indeed a substance, capable of existing independently of the body.

Thus the Essay entered the field at a time when an entire worldview was in crisis. It is hardly surprising that Locke's doubts about our ability to know the ultimate nature of reality should have occasioned so much resistance, for such humility flies in the face of the religious and scientific dogmatism that at once was responsible for the crisis in the first place and provided a haven for partisans.

This is especially clear in the denial of innate ideas, the thesis of Book I. For Locke, the mind at birth is an 'empty cabinet' that only experience can furnish; its materials are given by sensation and reflection. It is crucial to see that Locke's attack extends not only to innate ideas as Descartes understood them but also to intrinsic practical principles. The so-called 'Cambridge Platonists' had argued that certain moral principles are innate on the grounds that all human beings assent to them. Locke neatly disposes of this kind of argument by pointing out that universal assent does not entail innateness. (After all, a moral principle such as 'do not kill innocent people for pleasure' might be universally accepted simply because it is a valuable rule for any society to have.) In any case, the anthropological evidence, even as known to Locke, does not support the claim that all humans assent to such moral principles in the first place.

When we turn to Locke's arguments against Descartes, the issue becomes more complicated and more interesting. Descartes had posited a faculty, the intellect, which God stocked with ideas that allow us to understand the physical world, our own minds, and to a lesser degree, God himself. In the sixth of his Meditations, Descartes argues that our ability to demonstrate propositions about complex geometrical figures such as a chiliagon, or thousand-sided figure, shows that our mental powers are not limited to the information we receive from experience. Try to form a mental image of a chiliagon. Can you tell the difference between it and an image of a figure with 999 sides? What you come up with is at best a confused picture of a many-sided figure. Therefore there must be a purely intellectual idea, not derived from experience, that funds our demonstrations about such figures. Locke tackles this argument head on in II.xxix.13. The demonstrations Descartes made so much of are not in the first place about the chiliagon itself but rather about the numbers we assign to its sides and angles. Nothing more is required than an ability to perform simple mathematical operations. In Book I, Locke challenges the very coherence of innate ideas, the alleged objects of the intellect. Not even Descartes had claimed that we are always conscious of our innate ideas; these come to light only with effort. An innate idea, then, must somehow be 'in' the mind, even though we are not aware of it. Locke finds this absurd, for what is it to have an idea, other than to be conscious or aware of that idea?

It is all very well to deny innate ideas and principles. It is another thing altogether to invent a replacement for them. Can the empiricist reconstruct our knowledge of the world solely on the basis of experience? Locke turns to this task in the remaining books of the Essay, offering recipes for producing ideas of things, like God, that are remote from our experience. This brings into view the precise form of Locke's empiricism. He is not arguing that the simple passive reception of sensations constitutes knowledge; his claim is instead that the materials for knowledge must come from experience. We need not experience a unicorn to have an idea of it; but we must have experience of the constituent parts of that complex idea.

With this in place, Locke can deploy his "historical, plain method" (I.i.2) of inquiring into the mind and its contents as a weapon against dogmatists of every stripe. Human knowledge is limited to the ideas we receive either in sensation or in reflecting on what passes in our minds themselves. Although Locke is clearly sympathetic to the mechanical philosophy, he denies that it can show us the 'real essence' or nature of physical things. God has given us our senses to help us navigate through the world and discover His existence but has provided no means for knowing real essences. Descartes' dream was to give science a foundation that in no way relies on experience. Locke sees that this is impossible. The real job of philosophy is at once to remove the unjustified views that impede scientific progress and to rein in its frequent pretensions to having attained the ultimate truth of things.

When we turn from the physical world to the nature of the mind, we find that Locke is no less hostile to the Cartesian view that 'what thinks in us' is an immaterial substance. The fundamental nature of the mind is just as mysterious as that of the physical world. For all we know, the thinking part of a human being might be no more an immaterial substance than a heart or lung, a position that drew charges of atheism. But for Locke, whatever we need to regulate our lives and make moral judgments can be known without recourse to extravagant claims about the ontological status of the human mind. Thus in his celebrated discussion of personal identity (II.xxvii), Locke argues that whatever feature makes us numerically the same over time depends not on the continued existence of a substance but only on the continuity of consciousness available to introspection. An afterlife of rewards or punishments does not require the persistence of a Cartesian soul, only the continuation of stages of consciousness linked by memory.

What emerges from Locke's work is a solution to the problems of his age and, indirectly, our own. But his solution is skeptical rather than dogmatic: It seeks to confine our speculations to questions we can in principle answer rather than bogging down in disputes about meaningless words. We must think for ourselves, neither dismissing the deliverances of science nor taking them for unquestionable revelations. To this end, the Essay Concerning Human Understanding is an indispensable guide. As Locke says, "he who has raised himself above the alms-basket, and, not content to live lazily on scraps of begged opinion, sets his own thoughts on work, to find and follow truth, will (whatever he lights on) not miss the hunter's satisfaction."

A Note on the Text

This edition is that of A. C. Fraser (Oxford: Clarendon Press, 1894), which is based on a collation of the first four editions of the Essay and the French translation on which Locke worked with Pierre Coste. Spelling and punctuation have been modernized.

Walter R. Ott is an assistant professor of philosophy at East Tennessee State University. He holds a Ph.D. in philosophy from the University of Virginia and is the author of Locke's Philosophy of Language, as well as numerous articles on modern philosophy.