Ridling Zaine. Philosophy: Then and Now. A look back at 26 centuries of thought
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different sorts of epistemic notions. The concept of a disposition has itself
been further analyzed, for example by Roderick M. Chisholm (b. 1916), in
counterfactual terms, and it has been proposed by many philosophers that the
knowledge expressed by causal laws (laws of nature) is counterfactual and
thus dispositional in character.
A Priori versus A Posteriori Knowledge
A sharp distinction has been drawn since at least the 17th century
between two types of knowledge: a priori knowledge and a posteriori
knowledge. The distinction plays an especially important role in the
philosophies of David Hume (1711-76) and Immanuel Kant (1724-1804). It is
also found in many contemporary, empirically oriented theories of knowledge,
which typically hold that all knowledge about matters of fact derives from
experience and is therefore a posteriori and that in consequence such
knowledge is never certain but at most only probable.
The difference between these types of knowledge is easy to illustrate by
means of examples. Consider the sentences “All husbands are married” and
“All Model-T Fords are black” and assume that both statements are true. But
how does one come to know that they are true? In the case of the first, the
answer is that, if one thinks about the meaning of the various words in the
sentence, one can see that the sentence is true. One can see that this is so
because what is meant by “husband” is the same as what is meant by “married
male.” Thus, by definition, every husband is a married male, and, accordingly,
every husband is married. In calling such knowledge a priori, philosophers are
pointing out that one does not have to engage in a factual or empirical inquiry
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in order to determine whether the sentence is true or not. One can know this
merely on the basis of reflection and thus prior to or before any investigation
of the facts.
In contrast, the second statement can be determined to be true only after
such an investigation. One may well know that the Model-T Ford was an
automobile built prior to World War II and accordingly would understand
what all the words in the sentence mean. Nonetheless, understanding alone
would not be sufficient to allow one to determine whether the sentence is true
or not. Instead, some kind of empirical investigation is required in order to
arrive at such a judgment; the knowledge thus acquired is a posteriori, or
knowledge after the fact.
There are sets of distinctions related to the one just developed and in
terms of which the two propositions can also be differentiated. They are
necessary versus contingent, analytic versus synthetic, tautological versus
significant, and logical versus factual.
Necessary versus Contingent Propositions
A proposition is said to be necessary if it holds (is true) under all
possible circumstances or conditions. “All husbands are married” is such a
proposition. There are no possible or conceivable conditions under which this
statement would not be true (on the assumption, of course, that the words
“husband” and “married” are taken to mean what they ordinarily mean). In
contrast, “All Model-T Fords are black” holds in some circumstances (those
actually obtaining, and that is why the proposition is true), but it is easy to
imagine circumstances in which it would not be true – for instance, if
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somebody painted one of those cars a different color. To say, therefore, that a
proposition is contingent is to say that it holds in some but not in all possible
circumstances. Some necessary propositions, such as “All husbands are
married” are a priori (though not all are) and most contingent propositions are
a posteriori.
Analytic versus Synthetic Propositions
A proposition is often said to be analytic if the meaning of the predicate
term is contained in the meaning of the subject term. Thus, “All husbands are
married” is analytic because the term “husband” includes as part of its
meaning “being married.” A term is said to be synthetic if this is not so.
Therefore, “All Model-T Fords are black” is synthetic since the term “black”
is not included in the meaning of “Model-T Ford.” Some analytic propositions
are a priori, and most synthetic propositions are a posteriori. These
distinctions were used by Kant to ask one of the most important questions in
the history of epistemology, namely, whether a priori synthetic judgments are
possible
Tautological versus Significant Propositions
A proposition is said to be tautological if its constituent terms repeat
themselves or if they can be reduced to terms that do, so that the proposition is
of the form “a = a.” In such a case the proposition is said to be trivial and
empty of cognitive import. A proposition is said to be significant if its
constituent terms are such that the proposition does provide new information
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about the world. It is generally agreed that no significant propositions can be
derived from tautologies. One of the objections to the ontological argument is
that no existential (significant) proposition can be derived from the
tautological definition of “God” with which the argument begins. Tautologies
are generally known to be true a priori, are necessary, and are analytic; and
significant statements are generally a posteriori, contingent, and synthetic.
In the ontological argument, for example, God is defined (roughly
speaking) as the only perfect being. It is then argued that no being can be
perfect unless it exists; therefore, God exists. But, as Hume and Kant pointed
out, it is fallacious to derive a factual statement about the existence of God
from the definition of God as a perfect being.
Logical versus Factual Propositions
The term “logical” in this connection is used in a wide sense to include a
proposition such as “All husbands are married.” By analyzing the meaning of
its constituent terms one can reduce the proposition to a logical truth, e.g., to
“A and B implies A.” In contrast, factual propositions, such as “All Model-T
Fords are black,” have syntactical and semantic structures that differentiate
them from any propositions belonging to logic, even in the broad sense
mentioned above. The theorems of logic are often a priori (though not
always), are always necessary, and are typically analytic. Factual propositions
are generally a posteriori, contingent, and synthetic.
These various distinctions are widely appealed to in present-day
philosophy. For instance, Saul Kripke (b. 1941) in “Naming and Necessity”
(1972) has used these notions in an effort to solve a long-standing problem,
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namely, how true identity statements can be nontrivial. The problem, first
articulated by Gottlob Frege in “On Sense and Reference” (1893) and later
independently addressed by Russell, begins with the assumption that the
sentences “Scott is Scott” and “Scott is the author of Waverley” are both
identity sentences and are true and that the former is trivial while the latter is
not. The puzzle arises from the further assumption that any true identity
sentence simply says of some object that it is identical with itself. Hence, all
such sentences should be trivial. Clearly, however, “Scott is the author of
Waverley” is not trivial. But, if it is not, how is this possible?
Kripke argues that all true identity sentences are necessary (i.e., that they
hold in all possible worlds) and that some of these, such as “Scott is Scott,”
are known a priori and accordingly are trivial; but, he argues, some true
identity sentences are not known a priori but only a posteriori and are not
trivial. In cases of the latter sort, their nontriviality is a function of their being
known to be true only after some sort of inquiry or investigation. It is the
investigation that provides new information.
A good example would be the following. At one time in human history,
ancient peoples did not know that what they called “the evening star” was the
same planet called “the morning star.” But eventually the Babylonians
discovered through astronomical observation that the morning star is the
planet Venus as it appears in the morning sky and that the evening star is the
planet Venus as it appears in the evening sky. The discovery that these two
appearances are appearances of the same object amounted to discovering more
than that Venus is Venus. It provided new information, and that is why “the
morning star is identical with the evening star” is significant in a way in which
“Venus is Venus” is not, even though all of the descriptive terms in both
sentences refer to exactly the same object. In similar fashion, the a posteriori
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finding that it was Scott who wrote Waverley explains the nontriviality of
“Scott is the author of Waverley.” But no such investigation was needed to
determine that Scott is Scott.
Knowledge by Acquaintance and Knowledge by Description
The distinction between knowledge by acquaintance and knowledge by
description was introduced by Bertrand Russell in connection with his
celebrated theory of descriptions. Here only the epistemological (as distinct
from the logical) version of his theory will be considered. It was invented by
Russell to lend support to the basic thesis of empiricism that all knowledge of
matters of fact (i.e., all a posteriori knowledge) derives from experience.
Russell’s program is both reductive and foundationalist. It tries to show that
man’s system of knowledge is stratified: that some types of knowledge
depend on others but that some do not and that the latter form the foundational
units which give support to the whole epistemic system. He argued that,
because these basic units rest upon direct experience, ultimately all factual
knowledge is derivable from experience.
Russell’s argument begins with a distinction between two different types
of knowledge: that which is and that which is not based on direct experience.
Nearly all of man’s knowledge is of the latter type. For example, it is known
that some 2,000 years ago there lived a Roman statesman named Augustus,
that he was the successor to Julius Caesar, who had been assassinated, and
that he was a friend of the historian Livy. But, since none of these pieces of
information is presently known on the basis of personal experience, what
justification is there for calling them instances of knowledge?
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Russell argued that information based on direct experience is basic and
needs no justification; he called it “knowledge by acquaintance.” Information
not based on direct experience he called “knowledge by description.” One is
justified in calling such information knowledge, if one can show that it can be
traced back to and thus ultimately rests upon knowledge by acquaintance. To
show how this is so in a particular case is to legitimate that particular piece of
information as a specimen of knowing. Here is how this reductive process
would work in the case of what is known about Augustus.
Whatever information people in the 20th century have about Augustus
probably comes to them from literary works, such as Livy’s history of Rome.
Such information thus comes secondhand, via descriptions in books about the
life and activities of Augustus. But why call such descriptions knowledge?
The answer is that through a historical process one can trace such information
back to an original source like Livy, who was a contemporary of Augustus.
One learns, via this process, that Livy in his history of Rome is reporting
events that he had witnessed himself or that he had learned from other
eyewitnesses. One can call what he tells about Augustus knowledge, because
it is testimony that is based upon his or someone else’s direct experience.
Thus, knowledge by description is a legitimate form of knowledge, even
though it is ultimately dependent upon knowledge by acquaintance.
Russell’s reductive thesis then was that all legitimate specimens of
knowledge are either based upon direct experience or can be shown to be
dependent upon such direct experience via a chain of tight historical or causal
links. His theory was therefore a form of empiricism, because it tried to show
how all knowledge of matters of fact could be derived from experience.
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But there is a further feature of the theory, stemming from the empirical
tradition of John Locke (1632-1704) and David Hume, that gives a special
twist to the notion of “knowledge by acquaintance.” According to this
tradition, knowledge by acquaintance is always knowledge based upon what
Hume called “impressions,” or upon what Russell called “sense-data.” These
for Russell were mental entities that generally, but not always, reflected the
characteristics actually possessed by physical objects. But, unlike physical
objects, sense-data were the objects directly apprehended in an act of
perception. What Russell meant by “direct apprehension” or “direct
perception” was itself explicated in terms of the concepts of inference and
non-inference. He held that direct perception, i.e., the perceptual awareness of
a sense-datum, involves no inference and, accordingly, that knowledge by
acquaintance is identical with the perception of sense-data.
The difference between inferential and noninferential perception can be
illustrated by an example. Suppose one is working in a room and hears a
sound that emanates from an outside source. (Russell considered hearing to be
a form of perception.) In such a case the sound is a sense-datum. One need not
infer that one is hearing a sound; there is a direct awareness of it. This would
be a case of knowledge by acquaintance. On the basis of what one hears in
this direct fashion, one might then infer (guess, conjecture, hypothesize) that
what is causing the sound is a motorcycle located outside of the room,
something that one who is in the room cannot see directly. If one is correct in
this supposition, the information obtained in this way would be a case of
indirect knowledge. In such a case, one’s knowledge that there is a motorcycle
in the street is dependent on (and in Russell’s sense, reducible to) one’s direct
awareness of a sound. The example illustrates how indirect knowledge, such
as knowledge by description, is derived from direct knowledge, such as
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knowledge by acquaintance, and in turn how this latter depends upon the
direct awareness of sense-data.
It should be mentioned that the distinction between knowledge by
acquaintance and knowledge by description can be defended as legitimate and
useful independently of a commitment to sense-data theory. In Russell’s work
the objects of direct awareness are sense-data, but sense-data theory today has
few proponents. A philosopher thus might hold that one at least sometimes
directly perceives physical objects (which are not sense-data) while accepting
that one’s knowledge of past events and persons is indirect and is thus
knowledge by description.
Description versus Justification
Epistemology during its long history has engaged in two different sorts
of tasks. One of these is descriptive in character. It aims to depict accurately
certain features of the world, including the contents of the human mind, and to
determine whether these should count as specimens of knowledge. A
philosophical system with this orientation is, for example, the phenomenology
of Edmund Husserl (1859-1938). Husserl’s aim was to give an exact
description of the notion of intentionality, which he characterized as
consisting of a certain kind of “directedness” toward an object. Suppose the
object is an ambiguous drawing, such as the duck/rabbit sketch found in the
Philosophical Investigations of Ludwig Wittgenstein. A person looking at the
sketch is not sure whether it is a drawing of a duck or of a rabbit. Husserl
claimed that the light rays reaching the eye from such an ambiguous drawing
are identical whether one sees the image of a duck or of a rabbit and that the
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difference in perception is due to the viewer’s structuring of what he sees in
the two cases. The theory tries to describe how such structuring takes place,
and it ultimately becomes very complex in the account it gives.
In a famous passage in Philosophical Investigations (1953),
Wittgenstein states that “explanation must be replaced by description,” and
much of his work was devoted to carrying out that task, as, for example, in his
account of what it is to follow a rule. Another example of descriptive
epistemology is found in the writings of such sense-data theorists as Moore,
Price, and Russell. They begin with the question of whether there are basic
apprehensions of the world, free from any form of inference, and in those
cases where they have argued that the answer is yes, they have tried to
describe what these are and why they should count as instances of knowledge.
Russell’s thesis that the whole edifice of knowledge is built up from a
foundation composed of ingredients with which human beings are directly
acquainted illustrates the close connection between the attempt to characterize
various types of knowledge and this descriptive endeavor. The search by some
logical positivists, such as Moritz Schlick (1882-1936), Otto Neurath (1882-
1945), and A.J. Ayer (b. 1910) for protocol sentences, sentences that describe
what is given in experience without inference, is a closely related example of
this kind of descriptive practice.
Epistemology has a second function, which, in contrast to the descriptive
one, is justificatory or normative. Philosophers concerned with this function
start from the fact that all human beings have beliefs about the world, some of
which are erroneous and some of which are not. The question to them is how
one can justify (defend, support, or provide evidence for) certain sets of
beliefs. The question has a normative import since it asks, in effect, what one
ideally ought to believe. (In this respect epistemology has close parallels to