Plato: Meno

Here Plato explains how it is possible to have knowledge of things we never encountered or studied in this life. The doctrine of innate knowledge is the key. He believes that knowledge of the forms is already present at birth, obtained in a previous existence. To acquire knowledge, we simply have to recall what we already know.

Descartes: Meditations on First Philosophy (Descartes’s Doubt)

Descartes arrives at his initial skepticism via his famous dream and evil demon arguments. He introduces the dream argument by noting that “there are no certain indications by which we may clearly distinguish wakefulness from sleep.” Our dreams can seem like reality, and in dreams we often don’t know we are dreaming. So, it is possible that we are dreaming now, he says, and that what we take to be the real world is in fact not real at all. More to the point, it is possible that our sense experience—by which we presume to know material reality—is just a dream. If so, we can’t be certain about anything we think we know through our senses. Therefore, sense experience can yield no knowledge. Yet we seem to know even less than this argument suggests, Descartes claims. Suppose, he says, that “some evil genius not less powerful and deceitful, has employed his whole energies in deceiving me.” This being could delude us about every kind of experience we could possibly have. We can’t be sure that this is not the case. We can’t be certain that all our thoughts are not the work of an evil entity that infuses our minds with false sensations and ideas, making an external reality appear to exist. And if we are not certain of this, we can’t know anything based on experience.

Descartes: Meditations on First Philosophy (Descartes’s Certainty)

In the midst of profound skepticism, Descartes sees something that he cannot possibly doubt—he exists. If he can persuade himself of something, if he can have thoughts, he must exist. Even an evil genius cannot rob Descartes of this knowledge. In the very act of doubting, or of experiencing something contrived by the evil genius, Descartes finds unshakeable proof that he himself exists: “I think, therefore I am.” Moreover, he thinks he can know more than this: Anything that he perceives clearly and distinctly must be true. It must be true because God is no deceiver, and God ensures that it’s true.

Locke: An Essay Concerning Human Understanding

Locke debunks the idea of innate ideas and argues that the mind at birth is a tabula rasa, a blank slate. It is like white paper, devoid of characteristics until it receives sense perceptions. All knowledge begins with sensory experience on which the powers of the mind operate, developing complex ideas, abstractions, and the like. In place of the absolute certainty that the rationalists sought to find, Locke says that apart from the knowledge of the self, most of what we know we know in degrees of certainty derived from inductive generalizations.

Berkeley: Of the Principles of Human Knowledge

Berkeley claims that material objects do not exist; only sensations, or ideas, exist, along with the minds that perceive them. The world as we know it is nothing more than particular patterns of sense data. As he puts it, “To be is to be perceived.” Berkeley sets out to prove his theory by first trying to show that all the qualities we perceive are not in material objects; they are sensations in us. Material objects cannot exist because their existence would be logically absurd. The commonsense view is that material objects continue to be even when no one has them in mind. But, says Berkeley, this would mean that they can be conceived of as existing unconceived, that we can think about things that no one is thinking about—a logical contradiction. Therefore, Berkeley concludes, the claim that material objects exist is false.

Hume: An Enquiry Concerning Human Understanding

Like Locke, Hume locates the foundation of all our ideas in sensory experience. But Hume moves even further away from the possibility of absolute certainly of knowledge toward the view that we can justly have only relative certainty. We can be certain of only analytic truths (“relations of ideas”), namely, mathematics and tautologies. With regard to synthetic truths (“matters of fact”), we, at best, can have a high degree of probability. But even the notion of probability is dubious and leads to a certain skepticism because the notion of cause and effect on which experiential knowledge is based is itself not an impression but rather an idea.

Kant: Critique of Pure Reason

Instead of accepting the conventional view that knowledge is acquired when the mind conforms to objects, Kant maintains that objects conform to the mind. He argues that sense experience can match reality because the mind stamps a structure and organization on sense experience. Synthetic a priori knowledge is possible because the mind’s concepts force an (a priori) order onto (synthetic) experience.

Cole: Philosophy and Feminist Criticism

According to Cole, feminists believe that dominant knowledge practices disadvantage women by (i) excluding them from inquiry; (ii) denying them epistemic authority; (iii) denigrating their “feminine” cognitive styles; (iv) producing theories of women that represent them as inferior, deviant, or significant only in the ways they serve male interests; (v) producing theories of social phenomena that render women’s activities and interests invisible; and (vi) producing knowledge (science and technology) that is not useful for people in subordinate positions or that reinforces gender and other social hierarchies. Feminists have responded to these affronts in various ways. Feminist empiricism calls for a deeper, more rigorous application of empiricism. Feminist standpoint theory says that different social groups have distinctive kinds of knowledge acquired through unique experiences and that some of these groups may enjoy epistemological advantages over others. Feminist postmodernism is skeptical of such notions as objective or scientific truth, objective reality or fact, universal propositions, foundational knowledge, ultimate justification, and traditional conceptions of rationality.