Plato and Aristotle

UC Davis Philosophy 102

Theory of Knowledge

Fall, 2005

Instructor: G. J. Mattey, Senior Lecturer

Version 2.1, October 10, 2005

In the introductory remarks, I mentioned a number of important theorists of knowledge. In the second segment of the class, we will have a look at the main developments in the history of the theory of knowledge from ancient times to the present. The material will be presented in three modules. The present module covers the birth of epistemology in the work of Plato and Aristotle in the 4th century B.C.E. The second module is devoted to the contributions of the Hellenistic philosophers of the generations following Aristotle. The final module takes up the challenge posed by the ancient skeptics to the theories of Plato, Aristole, and the Hellenistic epistemologists.


Some philosophers before Plato, most notably Socrates, had made pronouncements about knowledge. (Socrates famously proclaimed that the only thing he knew was that he was ignorant.) But the origins of the theory of knowledge can be found in the dialogues of Plato, which set the tone for much of the shape of the projects I sketched out in the introductory remarks.

Plato's Methodology

As far as methodology goes, I shall make the case that Plato was a methodist in his treatment of knowledge. Plato also regarded knowledge as an objective property of human beings. One of the most important features of Plato's philosophy, a feature which he appears to have taken over from Socrates, is the view that when we attribute something to an individual (for example, knowledge), it is because we think that the individual falls under some universal characteristic. So a person is said to know in a particular instance because that person has fulfilled some specific condition or conditions required for there to be knowledge in that case.

The basic demand for an understanding of the universal characteristics of a kind of thing before we can make particular attributions about it can be found in the early dialogue Euthyphro. In the dialogue, the priest Euthyphro is prosecuting his father for impiety. Given the way this act violates the "family values" of the ancient Athenians, Socrates demands of Euthyphro that he state what it is that makes any given pious act pious.

And therefore, I adjure you to tell me the nature of piety and impiety, which you said that you knew so well, and of murder, and of other offences against the gods. What are they? Is not piety in every action always the same? and impiety, again--is it not always the opposite of piety, and also the same with itself, having, as impiety, one notion which includes whatever is impious?
Euthypro agrees and proceeds to give some examples of pious acts, as a particularist with respect to piety might. But Socrates demands more.
SOCRATES: Remember that I did not ask you to give me two or three examples of piety, but to explain the general idea which makes all pious things to be pious. Do you not recollect that there was one idea which made the impious impious, and the pious pious?
EUTHYPHRO: I remember.
SOCRATES: Tell me what is the nature of this idea, and then I shall have a standard to which I may look, and by which I may measure actions, whether yours or those of any one else, and then I shall be able to say that such and such an action is pious, such another impious.

This demand carries over to knowledge. We must look for a universal standard that applies to all cases of knowledge. Moreover, the standard is not to be found by examining particular cases, as a particularist would hold. This is why I regard Plato as having been a methodist (and indeed to have invented methodism itself). It is also clear in Plato that an adequate standard must be objective, and in the case of knowledge, an individual has it only if he or she meets this standard.

Plato's Analysis of Knowledge

Having described Plato's contribution to the methodological project, we can turn to the genesis of the analytical project in Plato. The most extensive discussion of the conditions for knowledge is found in the dialogue Theaetetus. It begins as one might expect. Socrates (who himself is very skeptical) is questioning Theaetetus, a bright young student.

Herein lies the difficulty which I can never solve to my satisfaction--What is knowledge? Can we answer that question?

Like most of those posed this kind of question, Theaetetus answered by giving examples.

Then, I think that the sciences which I learn from Theodorus-- geometry, and those which you just now mentioned--are knowledge; and I would include the art of the cobbler and other craftsmen; these, each and all of, them, are knowledge.
To this, Socrates gave his standard reply.
But that, Theaetetus, was not the point of my question: we wanted to know not the subjects, nor yet the number of the arts or sciences, for we were not going to count them, but we wanted to know the nature of knowledge in the abstract. Am I not right?
Socrates suggests that describing the nature of clay as being moistened earth is the right model for answering his question. Theaetetus is hesitant to attempt to apply this model to knowledge, because he thinks that the task of discovering the nature of knowledge in the abstract is much more difficult than even discovering the nature of mathematical objects. Socrates's reply is prophetic.
SOCRATES: And is the discovery of the nature of knowledge so small a matter, as just now said? Is it not one which would task the powers of men perfect in every way?
THEAETETUS: By heaven, they should be the top of all perfection!
In the lengthy course of the dialogue to follow, Socrates never does discover a satisfactory answer to his question. A look at some of the attempts to answer it will introduce some main themes of subsequent proposals for the analysis of knowledge.

Knowledge as Perception

The first proposal to be considered is that knowledge is perception. On this analysis, to be perception is both necessary and sufficient for knowledge. That is:

Keeping these two conditions separate will be helpful in understanding the objections to the analysis.

Theaetetus's first statement seems to be an inference from perception's being a necessary condition for knowledge to the conclusion that perception is identical or equivalent to (i.e., necessary and sufficient for) knowledge:

Now he who knows perceives what he knows, and, as far as I can see at present, knowledge is perception.
This proposal about the nature of knowledge is attributed by Socrates to the Sophist Protagoras, his older contemporary. It is subjected to intense criticism by Socrates and eventually rejected.

It will be useful first to note an ambiguity in the term "perception" (used to refer to perception through the senses). In the most general sense, to perceive is to "have perceptions." In this sense, I might perceive a ghost, even though there is no ghost before me, because there is an appearance of a ghost. In a more restrictive sense, to perceive something requires not only that there be an appearance of it, but that the action of the thing on the senses reveals the way the thing exists. In this "success" sense of the term, if I perceive something white, then the white thing causes the appearance of something looking white, through its interaction with my sensory apparatus.

The first set of objections is directed against the claim that perception is sufficient for knowledge. The first objection in that set concerns what has come to be called the relativity of perception.

The idea is that in some cases, such as being hot or cold, it seems that all perception is successful in that there is no distinction between perception and appearance. The reason is that hot and cold are nothing apart from their relation to the perceiver. To be hot or cold is for a person to feel hot or to feel cold: "things appear, or may be supposed to be, to each one such as he perceives them." This seems to be sufficient for knowledge in the case of hot and cold.

SOCRATES: Then perception is always of existence, and being the same as knowledge is unerring.

Note that Socrates has slipped into the conversation a standard of unerringness that seems to function as a necessary condition for knowledge.

The problem for the view is that perception is not always of existence, such as those that arise in dreams and in the minds of the insane. Theaetetus concedes this point.

I certainly cannot undertake to argue that madmen or dreamers think truly, when they imagine, some of them that they are gods, and others that they can fly, and are flying in their sleep.
If perception is not always of existence (if it is not always successful), it can be erring, and if perception can be erring, it fails to meet a condition that is necessary for knowledge. We will in what follows call cases of this kind "counter-examples." A counter-example fulfills a purported condition (an appearance of my flying is a perception, and hence by the proposed analysis a case of knowledge) but is not a case of what the condition is supposed to be a condition of (an appearance of my flying is not knowledge).

Socrates goes on to raise a further question, one whose significance would not be understood until Descartes took it up again in the seventeenth century.

How can you determine whether at this moment we are sleeping, and all our thoughts are a dream; or whether we are awake, and talking to one another in the waking state?
It seems that there is no way in which this can be done. The problem is supposed to discredit our faith in perception. Below we will see that Plato thought that knowledge is gained in a quite different way.

Another problem for the view of Protagoras is that each sentient being has knowledge that is no better than that of any other. No person is wiser than the rest. This is a personal dig at Protagoras, who charged great sums for students to learn from him. This theme is taken up later in the dialogue. The most important point for us here is Socrates's recognition that people generally believe that some people are knowledgeable and some are ignorant.

And are not we, Protagoras, uttering the opinion of man, or rather of all mankind, when we say that every one thinks himself wiser than other men in some things, and their inferior in others? In the hour of danger, when they are in perils of war, or of the sea, or of sickness, do they not look up to their commanders as if they were gods, and expect salvation from them, only because they excel them in knowledge? Is not the world full of men in their several employments, who are looking for teachers and rulers of themselves and of the animals? and there are plenty who think that they are able to teach and able to rule. Now, in all this is implied that ignorance and wisdom exist among them, at least in their own opinion.
THEODORUS: Certainly.
SOCRATES: And wisdom is assumed by them to be true thought, and ignorance to be false opinion.
Here the stage is set for a later analysis, according to which truth is a necessary condition for knowledge.

Having shown that perception is not sufficient for knowledge, Socrates attacks the analysis from the other direction, arguing that it is not necessary either. He claims that there are other avenues to knowledge besides perception, including memory. It seems that we know what we once perceived and now recall by remembering it. But given that perception (or in the case at hand, seeing) is the same as knowing, it follows, says Socrates that "not-seeing is not-knowing." But when we remember what we saw, we are not seeing, in which case we are not knowing.

Then the inference is, that a man may have attained the knowledge of something, which he may remember and yet not know, because he does not see; and this has been affirmed by us to be a monstrous supposition.
Neither Socrates nor Theaetetus makes what seems here to be an obvious emendation of the analysis, which is to allow that perception is necessary but not sufficient for knowledge. It could be held that if what is remembered was first perceived, then the could be knowledge.

In the dialogue itself, the conclusion is drawn that knowledge and perception are not equivalent or identical.

SOCRATES: Thus, then, the assertion that knowledge and perception are one, involves a manifest impossibility?
SOCRATES: Then they must be distinguished?
THEAETETUS: I suppose that they must.

After a number of digressions, Socrates turns to the question of whether something more than perception is required for knowledge. He contends that knowledge requires an understanding of "being" and other universals, and that this kind of understanding cannot be derived from perception.

Then knowledge does not consist in impressions of sense, but in reasoning about them; in that only, and not in the mere impression, truth and being can be attained?
Socrates goes further to claim that perception has "no part" in knowledge. The result is a thoroughly "rationalist" conception of knowledge:
we no longer seek for knowledge in perception at all, but in that other process, however called, in which the mind is alone and engaged with being.
The argument that perception plays no role in knowledge is rather quick and is open to objection.

True Opinion

The process in which the mind, apart from the senses, is engaged with being is now given a name: "thinking or opining." Now if knowledge is to be sought in that process, it might seem that knowledge is to be identified with thinking or having an opinion. But even Theaetetus cannot accept such an analysis.

I cannot say, Socrates, that all opinion is knowledge, because there may be a false opinion; but I will venture to assert, that knowledge is true opinion: let this then be my reply; and if this is hereafter disproved, I must try to find another.
What Theaetetus is proposing is an that there are two conditions necessary for knowledge, and that together they are sufficient.

Socrates devotes a good deal of time in trying to devise a satisfactory account of false opinion. In the process, he raises a number of interesting suggestions that were elaborated by later philosophers. One of them is that in thinking we are talking to ourselves. (Some philosophers nowadays postulate a "language of thought" in which mental discourse occurs.) Another is that there is in our mind something like a block of wax on which impressions are made. Some kinds of error can be accounted for by deficiencies in the impressions or the indistinctness of the perceptions which are to be matched to the impressions.

There is a suggestive analogy between the failure to match the two and the failure of an archer to hit his target with his shot. A further memorable analogy is that the mind is like an aviary, filled with birds. The "birds" are the things we have learned, knowledge we "possess" in one sense but do not "have" in another sense. False opinion arises when we seek to retrieve one kind of "bird" but mistakenly catch another kind instead.

The discussion finally moves forward when Socrates tries to refute the view that knowledge is true opinion. The problem stems from the fact that one can arrive at opinions through both teaching and persuasion. The defect of opinion is that it may have its genesis in the wrong place. Knowledge arises only when one is taught or has perceived what is known.

Opinion, on the other hand, whether it is true or not, can arise from sources other than teaching or perception. For example a judge could be persuaded, by a lawyer or an orator, to have a true opinion.

SOCRATES: When, therefore, judges are justly persuaded about matters which you can know only by seeing them, and not in any other way, and when thus judging of them from report they attain a true opinion about them, they judge without knowledge, and yet are rightly persuaded, if they have judged well.
THEAETETUS: Certainly.
SOCRATES: And yet, O my friend, if true opinion in law courts and knowledge are the same, the perfect judge could not have judged rightly without knowledge; and therefore I must infer that they are not the same.
True opinion is not sufficient for knowledge, and so knowledge is not equivalent or identical to true opinion.

True Opinion Plus Logos

The final attempt at an account of knowledge adds to true opinion the presence of "definition or rational explanation" (logos). This account is subjected to a series of criticisms that are somewhat difficult to comprehend. The basic idea is that none of three potential readings of "definition or rational explanation" is sufficient for an account of knowledge based on them.

In the course of the discussion, there is an argument that has a great deal of relevance to contemporary epistemology. Suppose that knowledge results from the addition of a "definition or rational explanation" to something we already possess but which we do not known what Socrates calls an "element." Yet it would seem that the elements themselves must be known: for how can knowledge emerge from the unknown?

Contemporary epistemology has yet to come to an agreement on this matter. Nowadays, we speak of the "foundations" rather than the "elements" of what is known. Some epistemologists deny Socrates's claim that the foundations of what is known are themselves known. If so, then it appears that there must be two different accounts of knowledge, one for the foundation, and one that rests on the foundation. On the other hand, one might agree with Socrates that the foundations are unknown (perhaps they are perceptions or beliefs). Then one must face up to the question of how we can build knowledge on a foundation which itself is unknown.

Socrates concludes the Theaetetus on a note of defeat.

And so, Theaetetus, knowledge is neither sensation nor true opinion, nor yet definition and explanation accompanying and added to true opinion?
Perhaps a more satisfactory account of knowledge can be found in another dialogue, Meno.


The descriptive project in epistemology is carried out largely in the dialogue Meno. The overall theme of the dialogue concerns the question of whether virtue can be taught. In the course of the discussion, Socrates tries to demonstrate that learning is not the result of the teacher communicating information to the learner, but rather of the elicting of knowledge that one already has.

Through a series of (leading) questions, Socrates is able to guide an uneducated slave boy into proving the Pythagorean theorem. The only explanation for the boy's spontaneous production of the proof is that he remembered it.

MENO: And I am certain that no one ever did teach him.
SOCRATES: And yet he has the knowledge?
MENO: The fact, Socrates, is undeniable.
SOCRATES: But if he did not acquire the knowledge in this life, then he must have had and learned it at some other time?
MENO: Clearly he must.
The since the knowledge came from some other time, Socrates reasoned, it must have come from a time before birth, which incidentally serves as a proof for immortality.

Plato's theory of recollection won few adherents in Western epistemology. It seems very much that Socrates did actually teach Meno the proof, in which case the argument for recollection falls away. Moreover, the doctrine of the pre-existence of the soul before birth is almost universally disregarded, especially during the Christian era, given the doctrine that the soul is created from nothing at the beginning of life. On the other hand, there is an alternative Plato did not consider, which is that the soul begins to exist already "pre-loaded," so to speak, with knowledge or at least with potential knowledge. This view is especially prominent in Descartes's theory of "innate ideas."

The Value of Knowledge

Later in the Meno, Plato makes some remarks about the value of knowledge, an issue still a topic of discussion. Socrates considers the question of whether knowledge is more useful than true opinion. If we think of the value of knowledge lies in its being a good guide, then there is no difference between the two.

SOCRATES: I will explain. If a man knew the way to Larisa, or anywhere else, and went to the place and led others thither, would he not be a right and good guide?
MENO: Certainly.
SOCRATES: And a person who had a right opinion about the way, but had never been and did not know, might be a good guide also, might he not?
MENO: Certainly.
SOCRATES: And while he has true opinion about that which the other knows, he will be just as good a guide if he thinks the truth, as he who knows the truth?
MENO: Exactly.
SOCRATES: Then true opinion is as good a guide to correct action as knowledge; and that was the point which we omitted in our speculation about the nature of virtue, when we said that knowledge only is the guide of right action; whereas there is also right opinion.
Meno objects that someone with knowledge is always right, while one with true opinion is sometimes wrong. Socrates rejoins that this is impossible, since it is agreed that the opinion is a true one. Nowadays it is common to rebut Socrates's rejoinder by saying that opinions that are actually true are false in "worlds" or situations which are very much like our world. Knowledge, on the other hand, will persist in those situations. In this way, true opinion is not so reliable a guide as knowledge.

Socrates locates the difference between true opinion and knowledge in the fact that true opinion is unstable, while knowledge is stable. It is a fact of life that our opinions come and go. What is needed to convert true opinion into knowledge is that it be "tied down" so that it is secure. Here Socrates makes a conjecture about how this fastening takes place, using an analogy with the mythical statues of Daedalus. Those statues fly away when not secured, but are very beautiful and valuable when they are.

What secures true opinion is recollection. When an opinion is connected to a remembrance of the soul's pre-life knowledge, knowledge is the result.

SOCRATES: Because they require to be fastened in order to keep them, and if they are not fastened they will play truant and run away.
MENO: Well, what of that?
SOCRATES: I mean to say that they are not very valuable possessions if they are at liberty, for they will walk off like runaway slaves; but when fastened, they are of great value, for they are really beautiful works of art. Now this is an illustration of the nature of true opinions: while they abide with us they are beautiful and fruitful, but they run away out of the human soul, and do not remain long, and therefore they are not of much value until they are fastened by the tie of the cause; and this fastening of them, friend Meno, is recollection, as you and I have agreed to call it. But when they are bound, in the first place, they have the nature of knowledge; and, in the second place, they are abiding. And this is why knowledge is more honourable and excellent than true opinion, because fastened by a chain.
Thus, the project of giving an account of knowledge, which was abandoned in the Theaetetus, is carried out in the Meno, though only tentatively. Socrates refused to commit himself to this account here, but he insists that there is a difference between knowledge and true opinion. "There are not many things which I profess to know, but this is most certainly one of them."

In Book V of the Republic, Plato elaborates on the difference between knowledge and opinion. Both are "faculties," one enabling us to know, and the other to form opinions. The faculty of knowledge is infallible, while the faculty of opinion is subject to error.

Next, it is claimed that the two faculties have distinct "spheres." The sphere of knowledge is clearly being, but it is difficulty to state what the sphere of opinion is. For non-being is the realm of ignorance, rather than opinion, and opinion is at least about something. Plato concludes that the sphere of opinion lies between being and absolute non-being.

The sphere of being includes "the absolute and eternal and immutable," which is the proper object of knowledge. Intermediate between it and non-being are the relative, perishing, and changing objects which make up the realm of sensory appearances. So, the objects of opinion are the ordinary objects of the world, and knowledge is confined to the superior realm of unchanging reality, of which appearances are but a dim copy. For example, beauty itself may be known, but of the beauty of physical things there can be only opinion.

Recollection Elaborated

It is in the dialogue Phaedo that Plato gives a detailed descriptive account of recollection. The account comes in the midst of a discussion of the immortality of the soul. Socrates tries to demonstrate that the soul must have some knowledge before birth, knowledge which is recovered through recollection. (He acknowledges, though, that this does not prove that the soul is incapable of being destroyed.)

The basic argument is that in order for us to judge the characteristics of sensible things, we need a precise standard of judgment. The example used in the dialogue is the equality in size of two objects. The standard of equality is absolute equality itself. Absolute equality is not to be found through the senses. Yet we apply the standard to things we do perceive and recognize that sensible equality falls short of absolute equality.

From the senses then is derived the knowledge that all sensible things aim at an absolute equality of which they fall short?
Then before we began to see or hear or perceive in any way, we must have had a knowledge of absolute equality, or we could not have referred to that standard the equals which are derived from the senses?--for to that they all aspire, and of that they fall short.
No other inference can be drawn from the previous statements.
And did we not see and hear and have the use of our other senses as soon as we were born?
Then we must have acquired the knowledge of equality at some previous time?
That is to say, before we were born, I suppose?

We acquire the knowledge of equality (as well as of beauty, goodness, justice, and piety) before birth, according to this story. At birth, this knowledge is forgotten. Later, upon the stimulation of perception, say in comparing the lengths of sticks, the forgotten knowledge is recovered.


The reader will notice that knowledge and opinion are given different treatment in different Platonic dialogues. A case can be made that Plato never really settled on any particular view about knowledge as being final, but instead continually came up with new conclusions when the issue arose in various contexts.

This has some bearing on the project of validation. At least as far as philosophical matters are concerned, Plato seems to have followed Socrates in thinking of survival in dialectic, the give-and-take of discussion, as the ultimate test of merit. Socrates seems to have found dialectic to be ultimately inconclusive. Some later followers of Plato thought that he himself was similarly skeptical. It would be an irony if the accounts of knowledge and opinion sketched by Plato's Socrates are themselves ultimately a matter of opinion.


Plato's student Aristotle is responsible for some very important developments in the theory of knowledge. Two of the most important of them are related to the normative project and to the descriptive project.


Aristotle really invented the normative project. He was the first to expound principles of logic. In the Prior Analytics, he produced a system of deductive logic known as "syllogistic."

A syllogism is discourse in which, certain things being stated, something other than what is stated follows of necessity from their being so. I mean by the last phrase that they produce the consequence, and by this, that no further term is required from without in order to make the consequence necessary. (Book I, Chapter 1)
Aristotle investigated syllogisms abstractly, looking only at the form of what is stated. We do not have time here to investigate the details of Aristotle's syllogistic. (For details, see a nice summary by Professor Marc Cohen.)

A second book of logic, the Posterior Analytics, deals directly with the topic of knowledge. Here, Aristotle draws a number of distinctions which frame much of the discussion of knowledge in the present day. We will look at some of the high points of this seminal work.

The book begins with the assertion that "All instruction given or received by way of argument proceeds from pre-existent knowledge" (Book I, Chapter 1). Thus, Aristotle is focusing on a means of producing new knowledge from already existing knowledge, what we now might call the "transmission" of knowledge through argument.

Aristotle recognized several types of argument. One kind is mathematical demonstration. Another kind is "dialectical," and a third type is "rhetorical." The main interest in the book is with dialectical arguments. These are of two types: syllogistic, and inductive. We have already looked briefly at syllogistic arguments. They begin with at least one general premise. Inductive arguments, by contrast, begin with a specific premise and draw a general conclusion. The premises of an syllogistic argument are the "pre-existent" knowledge accepted by anyone reasoning from them. In the case of inductive arguments, the pre-existent knowledge is something particular, which is "exhibiting the universal as implicit in the premises" (Book I, Chapter 1).

Scientific Knowledge

From the beginning, Aristotle acknowledged that knowing can be understood in a qualified and in an unqualified sense. He used this distinction to try to overcome an aspect of Plato's problem of the Meno. Suppose I know the truth of the Pythagorean theorem. How could I ever learn the length of the hypotenuse of a triangle I have never encountered, as it would seem that I already knew it through my knowledge of the theorem? The solution is that I did not know the specific instance in an unqualified sense, but only knew it in a qualified sense through my knowledge of the theorem.

Having made this distinction, Aristotle turns his attention to unqualified scientific knowledge, which he distinguishes from the accidental, unscientific, knowledge that one might get through persuasion. Scientific knowledge requires that the cause of the fact in question be known, that it be known that the cause is of that fact alone, and that the fact is (in some sense) necessary.

We suppose ourselves to possess unqualified scientific knowledge of a thing, as opposed to knowing it in the accidental way in which the sophist knows, when we think that we know the cause on which the fact depends, as the cause of that fact and of no other, and, further, that the fact could not be other than it is.
Much of the rest of the book is devoted to how we can satisfy these conditions on scientific knowledge.

If knowledge is gained by syllogistic reasoning, there is said to be a "demonstration." The key to the syllogistic demonstration is the status of the premises.

Assuming then that my thesis as to the nature of scientific knowing is correct, the premises of demonstrated knowledge must be true, primary, immediate, better known than and prior to the conclusion, which is further related to them as effect to cause. (Book I, Chapter 2)


We shall not look at all these conditions. But we can say that what is particularly important is Aristotle's claim that there must be some basic, indemonstrable premises if there is to be scientific knowledge at all. This claim is his solution to a problem that has come to be known as the "regress" problem. The regress problem is to this day a prominent subject of the literature of epistemology.

Let us begin with the claim that all knowledge is demonstrative. If so, then for any known conclusion, there are premises prior to it and which are better known than it. Since they are known, they are themselves conclusions of demonstrations, and thus there are premises prior to and better known to them. These conditions continue to apply as far back as we push the chain of demonstrations, and an infinite regress is generated. Aristotle agrees that it is impossible to complete an infinite chain of demonstrations. So, if all knowledge is demonstrative, then there is no knowledge.

Skepticism can be avoided by allowing that the chain of demonstration is circular, that it at some point loops back on itself. Aristotle rejected circular demonstration for two reasons. The first involves the condition that the premises must be prior to and better known than the conclusion. If circular reasoning is allowed, then a conclusion will be a premise for itself, and hence prior to and better known than itself, which is impossible. The second objection is that "their theory reduces to the mere statement that if a thing exists, then it does exist--an easy way of proving anything" (Book I, Chapter 3).

So, if there is to be scientific knowledge at all, then there must be indemonstrable or primary premises. These premises are necessary truths, since their conclusions are necessary as well. They are universal in scope. Their main substantive feature is that they attribute essential properties to their subject. Because the conclusions are necessary, there can be no scientific demonstration of chance conjunctions. One of the primary criticisms of Aristotle's account of demonstrative knowledge has been that one can gain no substantive knowledge of facts from the primary premises, which really amount to nothing but definitions.

Knowledge and Opinion

Aristotle explains the difference between scientific knowledge and opinion on the universality and necessity of the former.

[I]t is opinion that is concerned with that which may be true or false, and can be otherwise: opinion in fact is the grasp of a premiss which is immediate but not necessary. This view also fits the observed facts, for opinion is unstable, and so is the kind of being we have described as its object. Besides, when a man thinks a truth incapable of being otherwise he always thinks that he knows it, never that he opines it. He thinks that he opines when he thinks that a connexion, though actually so, may quite easily be otherwise; for he believes that such is the proper object of opinion, while the necessary is the object of knowledge. (Book I, Chapter 33)
The appeal to necessary connection to distinguish knowledge and opinion allows Aristotle to be less radical than Plato, who claimed that knowledge is of being and opinion concerns a mere shadow of being. For Aristotle, the objects of knowledge and opinion are the same. What differs is what is said about the objects: whether they must be in the way they are thought or whether they might be otherwise.

Intuitive Knowledge

Many pages are devoted to the details of scientific demonstration. These discussions are part of what I call the normative project. Toward the end of the book, Aristotle turns to the consideration of primary premises and raises a number of issues that concern the descriptive project.

But there are questions which might be raised in respect of the apprehension of these immediate premisses: one might not only ask whether it is of the same kind as the apprehension of the conclusions, but also whether there is or is not scientific knowledge of both; or scientific knowledge of the latter, and of the former a different kind of knowledge; and, further, whether the developed states of knowledge are not innate but come to be in us, or are innate but at first unnoticed.
He rejects the claim that there are innate states of knowledge on the grounds that they would be "more accurate" than the conclusions of demonstration yet are unnoticed (Book II, Chapter 19). In order to be able to acquire these states of knowledge, we need to have a capacity which allows this. The fundamental capacity, common to all animals, is sense-perception. More advanced animals have memory as well. Repeated memory give rise to experience.

This raises a difficult question. How can experience, which is based ultimately on sense-perception, give rise to knowledge of necessary truths, of facts which cannot be otherwise? The answer Aristotle gives is that after enough repetition of the association of a subject and an attribute, the universality of the attribute is recognized.

It is like a rout in battle stopped by first one man making a stand and then another, until the original formation has been restored. . . . When one of a number of logically indiscriminable particulars has made a stand, the earliest universal is present in the soul: for though the act of sense-perception is of the particular, its content is universal-is man, for example, not the man Callias. A fresh stand is made among these rudimentary universals, and the process does not cease until the indivisible concepts, the true universals, are established: e.g. such and such a species of animal is a step towards the genus animal, which by the same process is a step towards a further generalization.
Aristotle calls this process of grasping the universal "induction." Once the universal is grasped, we use "intuition" to recognize the truth of the primary premises. This is the end of the story for Aristotle, but the idea that knowledge begins with intuition of truths that cannot be demonstrated has been a powerful one in the history of epistemology and remains so today.

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