Background to Locke's Theory of Knowledge

by Dr. Jan Garrett

last revised date: February 11, 2004

John Locke: 1632-1704

John Locke's two major works, the Essay Concerning Human Understanding and Two Treatises of Government, were published in 1689 and 1690, respectively.

During the 17th century the modern scientific outlook associated with Copernicus, Galileo and Isaac Newton had established itself as a dominant intellectual force in Europe, replacing the natural philosophy associated with Aristotle and the geocentric astronomy associated with Ptolemy. Descartes can be seen as a crusader for the modern scientific outlook, which regards nature as a complex machine to be understood according to mathematical natural laws.

By the time Locke publishes his major works, the authority of the modern scientific movement is well established, at least in England. The best sign of this is the 1687 publication of Isaac Newton's Mathematical Principles of Natural Philosophy or Principia three years before Locke's two major works appeared. Newton's Principia became the Bible of the scientific world for at least a century.

One can make sense out of Descartes' writings without much reference to the political events of his day, but this is less so with Locke. England endured several major political upheavals in Locke's lifetime. He was born under the absolute monarchy of Charles (Stuart) I. The rising middle class, linked to expanding commerce and supported by more egalitarian forms of Protestantism, came to dominate Parliament. The Parliament challenged the power of Charles (Stuart) I, and the inability of the two sides to work out a modus vivendi led to Parliament's ordering Charles I beheaded in 1649. England was then without a monarch for a number of years. It was ruled by Parliament for a number of years and then by Oliver Cromwell, who had been the leader of Parliament's military forces against those fighting to restore the Stuart monarchy.

The absolute monarchy was eventually restored under Charles (Stuart) II, which eventually died and was succeeded by his son James II. The restoration did not prove satisfactory.

Locke was reared in a liberal Puritan family and educated at Oxford. He then held various teaching and diplomatic posts. He was associated with Lord Ashley, Earl of Shaftsbury, who eventually became leader of parliamentary opposition to the restored Stuart monarchy. Shaftsbury was tried for treason in 1681, but acquitted; Shaftsbury went into exile in Holland and Locke followed two years later.

In 1688 a relatively nonviolent revolution -- so-called Glorious -- permanently removed the Stuart monarchy and established a constitutional monarchy, a development obviously welcomed by Locke and to some extent defended in his major works.

The political struggles of the 17th century were tied up with religious conflicts. In the Thirty Years War (1618-48) war between shifting alliances of Catholic, Lutheran, and Calvinist states devastated central Europe, setting civilization back about two generations in some parts. In England religious differences played a major role in the struggle between Parliament, led by Calvinists, and the Anglican royalist party surrounding the King. Catholicism is involved too, since the British were involved in making a colony of Ireland, where most natives were Catholic, and England's major rivals are associated with Catholicism.

Like Descartes in the previous century, Locke is concerned with knowledge. He is engaged in epistemology, a philosophical study of the nature, extent, and degree of certainty of knowledge. As Kant will eventually describe the key question of one of his own major works, the question that dominates the Essay Concerning Human Understanding is "What can I know?"

But when Descartes wrote in the early 17th c., it still to him seemed as if the religious and moral traditions in which he grew up did not need the same kind of reform from the bottom up as physics and metaphysics.

But Locke is more concerned with issues regarding moral and religious belief. And he is aware that there are several versions of Christianity, each of which claims to be true religion, and that this variety is not going to disappear. One used to be able to resolve disputes by appeals to tradition, and to the interpreters of tradition centered in the Catholic Church. But these are no longer authoritative for most Englishmen. (And Locke holds, anyway, that tradition and human religious authorities are no absolute guarantee of truth.)

Direct appeal to Scripture won't always help because different sects understand Scripture differently, and lacking a single accepted institutional "court of appeal," which the Pope once but for Englishmen no longer provided, there is no obvious mechanism to resolve disputes.

Locke offers his theory of knowledge as a way to use the shared human capacity, reason, to decide how much rational assent a person should give to at least some propositions or claims. Unlike Descartes, Locke was more concerned with religious and moral propositions as with mathematics or physical science.

Locke is also motivated by a political goal: the service of religious tolerance (he wrote an important shorter discourse on this subject, his Letter on Tolerance; Locke believes that too much blood has been shed over matters of faith. He distinguishes between what can be known and what must be taken on faith. And in his view neither knowledge nor faith require the use of force against persons who intellectually or religiously disagree. For more on this topic see Locke on Reason and Faith.

According to Locke, his Essay Concerning Human Understanding will inquire into the origin, extent (scope), and (degree of) certainty of human knowledge. In short, it is a work of what we now call epistemology.

Locke's method. Like many philosophers and thinkers in other fields, Locke makes use of a method we could call the method of analysis and synthesis. It follows the strategy: to understand something complex, mentally break it up into its component parts, until you come to something simple enough to grasp clearly. Then study how these simple parts connect with each other to form the complex whole.

This is the same method that the ancient atomist philosophers Democritus and Epicurus used to comprehend nature. Incidentally, around this time interest in the ancient philosophy of atomism revived. Atomism is one way, though not the only way, that one can develop a mechanical philosophy of nature along the lines of modern physics. Locke himself is attracted to this idea, which he refers to as the corpuscular hypothesis. This side of Locke would have been more pronounced if he weren't so skeptical -- or so doubtful -- regarding our ability to know material substances as they essentially are in themselves.

Locke devotes his method of analysis and synthesis primarily to his study of our mental life, our ideas and mental powers. Hence the phrase in the title of his work "concerning Human Understanding."

Like Descartes, Locke is currently regarded by most historians of philosophy as a foundationalist in epistemology. Foundationalism is characterized by the concern--some would say the anxiety--to establish knowledge or science upon a firm and unshakable foundation (the phrase "unshakable foundation" is from Descartes), as if any system of ideas without such a foundation would be a "castle built upon sand" (another phrase from Descartes) ready to fall over at the first puff of intellectual wind, in short, unreliable.

Foundationalism amounts roughly to this: some of the ideas we have, or can get, can be distinguished by certain inherent features as most secure and least doubtful. For Descartes these features were clarity and distinctness, and he thought they were found in basic mathematical truths, basic philosophical truths such as "minds are mental substances," and in the certainty we each have of our own existence.

Locke's foundationalism is expressed in the passage on MP, p. 280-81 (Book II, chapter 1.24).

The ideas gotten by sensation . . . are the impressions that are made on our senses by outward objects that are extrinsic to the mind and its own operations, proceeding from powers intrinsic and proper to itself, which when reflected on by itself, become also objects of its contemplation, and are . . . the origin of all knowledge. Thus the first capacity of human intellect is that the mind is fitted to receive the impressions made on it either through the senses by outward objects or by its own operations when it reflects on them. This is the first step a man makes towards the discovery of any thing and the groundwork on which to build all those notions which ever he shall have naturally in this world (my emphasis--JG) . . . . In all that great extent in which the mind wanders, . . . it does not stir one jot beyond those ideas which sense or reflection have offered for its contemplation.
A foundationalist philosophy proceeds to construct itself by building upon secure and somehow immediately evident premises which it discovers. It tries to build upon these premises in a strict logical fashion, preferably deductively, with reasoning like that which characterizes the theorems of geometry. The earlier points become the foundation for later points.

Locke places much greater emphasis upon direct observation of the internal life of one's own soul. See also Book II, chap. 12.15-16 (not in MP):

the best way to come to truth being to examine things as they really are, and not to conclude they are, as we fancy of ourselves, or have been taught by others to imagine . . . . I can speak but of what I find in myself.
Descartes and Locke are frequently paired as founders of the two main traditions of the foundationalist philosophy of the Enlightenment, Descartes as a rationalist and Locke as an empiricist. It is sometimes added that rationalists start from clear and distinct a priori ideas, ideas we get without reference to sensory experience; while empiricists start from ideas given in sensory experience.

A warning, however, is in order. Suppose we define rationalism as "building a system of knowledge by means of careful deductive reasoning" rather than "building a system of knowledge by use of a priori starting points," Locke is almost as much a rationalist as Descartes. Of the three kinds of knowledge that Locke admits (in book IV of the Essay), only one type -- sensitive knowledge -- involves knowing about the external world, and Locke holds that it is much less secure than "demonstrative" reasoning -- his term for deductive reasoning -- from self-evident premises.

And what of Locke's empiricism--his view that we can gain knowledge by means of the senses? We are going to find that he regards such knowledge as extremely limited. It does not extend, for example, to the essence or internal constitution of material substances.

Locke's emphasis on epistemology, his concern for drawing the line between what we can know and what we cannot know, is inseparable from his challenge of at least one doctrine associated with the Aristotelians, namely, that we can know what various substances are in themselves, in their real essences.

Locke is a critic of Aristotelianism as he understands it. Yet his Essay cannot be understood without a grasp of several Aristotelian concepts that became part of general philosophical discourse in the centuries that preceded Locke. These are the same ideas regarding Categories, essence, accident, and property that we discussed earlier. See Background to Modern Philosophy. They also include the term substance.

Philosophers of nature who reject other aspects of Aristotelianism continue to use this conception of the nature or essence of things vs. their more accessible but less explanatory features called properties; they continued to believe that knowing the essences of things

(i) is possible,
(ii) permits us to deduce the properties of things,
and (iii) is intellectually superior to merely knowing properties.
In his Metaphysics Aristotle distinguishes such senses of substance as
(1) the matter of a thing;
(2) the form or essence of a thing;
and (3) the composite of (1) and (2), the concrete individual which he had called primary substance in the Categories.
Descartes and his followers such as Malebranche tend to split human beings into two substances, minds and bodies. It's as if we are two concrete individuals, a mind (thinking substance) and a body (corporeal substance). Like Descartes, Locke is a dualist on what is called the mind-body question; the mind is not a part of the body, not something physical, not the nervous system or brain.

I mention this now because Locke will raise the question whether we can know the essence of bodies (his favorite example is the essence of gold, presumably much simpler than the human body) and whether we can know the essence of our mind or soul. This is a natural question to ask when the Aristotelian concepts of substance and essence have been reinterpreted for use against the background of dualism, a commitment to mental and corporeal substances.

A final aspect of philosophical background. Locke's philosophy seems to be influenced at key points by a medieval metaphysical tradition sometimes called Nominalism but more properly known as Conceptualism. Let us define both these terms. To do that we have to talk about two other isms, Extreme and Moderate Realism.

These four isms are answers to the question: what sort of reality do the general things (what Aristotle called species and genera) so important to scientific and metaphysical knowledge have.

The Medieval Platonist answer is that these things are the Forms or Ideas. They exist apart from particulars and they are most real. Humanity exists apart from particular humans and is more real than the particulars are. The particulars are sort of copies of the Form. This view is sometimes called Extreme Realism. (The Platonic form or idea is a universale ante rem.)

The Aristotelian answer, sometimes called Moderate Realism, is that the general things, or universals, exist in two ways, first as forms or general structures within particulars and common to particulars of a given species; second as general ideas in the mind of persons who know those forms. Thus humanity exists in particular human beings and in the idea of the philosopher who knows what it is to be human (universale in re and universale in mente).

By contrast with such extreme and moderate realisms, Nominalism is the view that there are only individual things and that such entities as species and genera (human and tiger, animal and plant) do not exist except as mere words or puffs of air (nuda nomina, flatus vocis) referring to similarities among individuals. Thus, for a nominalist, there is no human nature, just Tom, Dick, Harry, and Sally, who happen to resemble one another in various ways.

Conceptualism holds that species and genera are concepts or abstract ideas created by the human mind after experience with the thing (a concept is a universale post rem); our knowing makes use of the general concepts our minds have created, but these concepts do not refer to general things existing outside mind.

Locke's view of abstract ideas seems very close to conceptualism. In attacking Locke's position on abstract ideas, Berkeley and Hume seem to adopt a nominalist position.


1. "Universale ante rem" literally means the universal prior to the (particular) thing.

2. "Universale in re" literally means the universal in the (particular) thing; "universale in mente" means the universal in the mind (i.e., after it has been abstracted from similar particulars).

3. "Universale post rem" literally means the universal created (by the mind) after (experience of) a set of similar particular thing(s).