The problem with relying on reason alone, though, is that a priori propositions (that is, propositions whose truth or falsity is known prior to and independent of any sense experience) do not provide any useful information about the world. To know, for example, that bachelors are unmarried, that unicorns have horns, or that triangles have three sides, does not tell us whether there are such things in the world as bachelors, unicorns, or triangles. For that information, we have to rely on experience. But if experience is ruled out by rationalists as unjustified bases for knowledge, then it seems that we will never be able to know anything about the world.
Descartes tries to get around this problem by saying that we know that we exist, and that is some information about the world that is not based on sense experience. We also know (he says) that God exists and that God does not deceive us when we limit our beliefs about the world to clear and distinct ideas. So we can know things about the world insofar as it is clearly and distinctly organizable. That last feature means that, if we think about things in the world not in terms of what we learn from relying on our senses (e.g., that grass is green, or that it is cold outside) but only in terms of how things have to be (e.g., that in order to be grass, a plant has to have certain characteristics), we will then be able to say that we know something about them. Otherwise, we have to admit that we have beliefs about things, not knowledge.
Such a conclusion simply seems wrong to those who think that we are justified in trusting sense experience as the source of knowledge. Such thinkers are called empiricists (from the Greek empeiria , experience) because they claim that all knowledge of things in the world is based ultimately on experience, not on the purely mental operations of reason alone. For empiricists, facts about the world are known a posteriori (that is, they depend on experience for their truth or falsity), are publicly verifiable, and exhibit enough order that they can be the basis for generalizations and predictions. Admittedly, experience does not provide the absolute certainty that rationalists require for saying that you know something. But even if our knowledge is only probable, at least it is of some use (rather than being simply about how we understand definitions).
II. John Locke (1632-1704) is an example of a classical empiricist. He denies Descartes' claim that we are all born with certain innate ideas (e.g., self, God) or beliefs (e.g., A is identical to A, something cannot be and not be at the same time in the same respect); rather, we learn these things through experience or through thinking about what we experience. We start out at birth with a "blank slate" (tabula rasa ), and through experience we learn how and what to think about the world.
Even though we have access immediately only to our ideas, we can know things about the world because some of our ideas accurately represent how things are apart from how we think of them: this position is called "representational realism." For example, our ideas about whether things are solid, take up space, have shape, motion, or are one or many actually have a basis in reality: they are ideas of the "primary qualities" of things. Ideas of "secondary qualities" (e.g., colors, sounds, smells, tastes, textures) are caused in our minds by primary qualities such as the shape, movement, or molecular structure of things in the world. Ideas of secondary qualities represent things in the world not as they are but only as they appear to us. As long as we limit claims about the world to ideas of primary qualities, we can rely on our experience for knowledge.
Objections to Locke :
III. David Hume (1711-1776) extends the empiricist project by insisting that knowledge is based on perceptions. Perception includes impressions (sensations, passions, and emotions) and ideas (the faint-image copies or derivatives of impressions). All ideas must be traceable to some impression; otherwise, they are meaningless. Memories are less vivid impressions; and ideas of impressions called up at will are the product of the imagination. Where we have no impression, we have no idea and cannot claim to know that such things exist. So we cannot know that there is any such thing as our "self," since we experience impressions, sensations, and emotions but never the thing that has those experiences.
In a similar way, we think that we know that all events have causes. But when we try to identify trace such knowledge back to experience, we discover that all we mean by saying that A causes B is that A occurred before B, A seems to be near B in space and time, and that in our experience events like A seem to be followed with some regularity by events like B. The problem with this, Hume notes, is that we do not experience a necessary connection between A and B; we simply have a natural inclination to assume that there is a connection (a "constant conjunction") between an event and its supposed cause. There is no philosophic (empirical) justification for thinking every event has a cause; and even if every event in the past has had a cause, that is no justification for thinking that future events will have causes as well, since we do not know that future events will resemble past events. So even the assumption that there is a high probability that things in the furture will resemble the past, like the assumption that our present experiences are linked to our past experiences by means of some "self," is empirically groundless and philosophically suspect.
We can certainly believe that selves and causality are real, but such beliefs are products of imagination rather than knowledge. The only things we can know with certainty, then, are those things we are immediately experiencing now. And if that means that all we can say is something like "right now, I know that I am perceiving what I think is fire," that does not tell us much more about the world (though it might tell me something about my perception of the world) than the a priori propositions favored by the rationalists.
Objections to Hume :
IV. This debate between empiricists and rationalists prompts Immanuel Kant (1724-1804) to highlight differences between the kinds of statements, judgments, or propositions that guide the discussion. The kinds of propositions that rationalists use as models for knowledge--logical statements such as "A whole is always greater than any one of its parts" and mathematical statements such as "Triangles have three sides"--are matters simply of definition; they provide us with no real knowledge other than regarding what terms mean. As long as we understand the meaning of a certain term, we can analyze it (that is, unpack what is implicit in the term) without learning anything about whether there is anything in the world that that term describes. Statements that identify characteristics that are already implicit in the meaning of a concept or object (such as "all bodies take up some space," "bachelors are human beings") are thus called analytic statements or propositions. To put this more formally: true analytic judgments are those in which the predicate (e.g., being unmarried) is contained within the subject (e.g., bachelor); and if you try to deny such a proposition (such as saying, "It is not the case that bachelors are unmarried"), you contradict yourself.
In contrast to analytic propositions, there are other propositions in which the predicate (that is, what you say about something) is not part of the meaning or definition of the thing. To say, for example, that some birds are yellow is to say something about birds which is not contained within the definition of what it is that makes a bird a bird. After all, not all birds have to be yellow. By adding this new bit of information, you are combining or synthesizing two ideas, one of which (yellow) is not already implicit within the meaning of the other (bird). A proposition such as "Some birds are yellow" is therefore called a synthetic proposition. Synthetic propositions are statements in which the predicate is not contained within the subject; and if you deny such a proposition (such as saying, "It is not the case that some birds are yellow"), you do not necessarily contradict yourself.
The distinction between a priori judgments and a posteriori judgments is not the same as that between analytic and synthetic judgments. The distinction between a priori and a posteriori judgments deals with whether you have to rely on experience to determine whether the proposition is true or false. The distinction between analytic and synthetic judgments deals with whether what you say about a thing is already contained in the meaning of the thing. Since analytic judgments can be made in most cases without having to appeal to experience, they happen to be a priori judgments as well; just as in most cases, synthetic judgments happen to be a posteriori judgments.
However, for Kant, the distinctions between analytic and synthetic and a priori and a posteriori judgments must be kept separate, because it is possible for some judgments to be synthetic and a priori at the same time. What Kant proposes is this: Surely all a posteriori judgments are synthetic judgments, since any judgment based solely on experience cannot be derived merely by understanding the meaning of the subject. But this does not mean that all synthetic judgments are a posteriori judgments, since in mathematical and geometrical judgments, the predicate is not contained in the subject (e.g., the concept 12 is not contained either in 7, 5, +, =, or even in their combination; nor does the concept "shortest distance between two points" contain the idea of a straight line). Such propositions are universal and necessary (and thus a priori ) even though they could not have been known from experience; and they would be synthetic a priori judgments.
If there are such judgments, then how are they possible? Kant's answer: the rationalists are right in saying that we can know about things in the world with certainty; and the empiricists are right in saying that such knowledge cannot be limited merely to truths by definition nor can it be provided by experience. Instead, we know about the world insofar as we experience it according to the unchanging and universally shared structure of mind. All rational beings think the world in terms of space, time, and categories such as cause and effect, substance, unity, plurality, necessity, possibility, and reality. That is, whenever we think about anything, we have to think about it in certain ways (for example, as having causes, as existing or not existing, as being one thing or many things, as being real or imaginary, as being something that has to exist or doesn't have to exist), not because that is the way the world is, but rather because that is the way that our minds order rational experience.
We can be said to know things about the world, then, not because we somehow step outside of our minds to compare what we experience with some reality outside of it, but rather because the world we know is always already organized according to a certain fixed pattern that is the mind. Knowledge is possible because it is about how things appear to us, not about how things are in themselves. Reason provides the structure or form of what we know, the senses provide the content.
Objections to Kant :