Adler on Knowledge and Opinion

by Mortimer J. Adler, Ph.D.

Question: Is there such a thing as knowledge, or is everything a matter of opinion? Our picture of the world and our way of life has changed so much in the last fifty years that I wonder whether we can have certain knowledge about anything. Isn’t most of our so-called knowledge really opinion?

Dr. Adler: Most of us know what an opinion is. We recognize that our opinions are beliefs that others need not share. We are used to having those who disagree with us say, “Well, that is only your opinion” (or “your opinion”). Even when we advance an opinion on very good grounds, we usually feel some doubt about it. “I have good reason to believe so,” we say, “but I wouldn’t swear to it.”

Here, then, are three characteristics of opinions: (1) they express probabilities rather than certainties; (2) they are subject to doubt; and (3) reasonable men can differ about which of two conflicting opinions is sounder.

There is a perennial skepticism which holds that everything is a matter of opinion. The extreme skeptic reduces even such things as mathematics and science to opinion. He points out, for example, that a system of geometry rests on arbitrary assumptions. Other assumptions can be made and other systems of geometry developed. Experimental science at it’s best, the skeptic maintains, consists of highly probable generalizations, not indubitable certainties.

In contrast with such skepticism is the view of the ancient Greek philosophers. Plato and Aristotle think that there are some matters about which men can have genuine knowledge. In the very nature of things, some things are necessary and cannot be otherwise. For example, by the very nature of wholes and parts, it is necessary that the whole should always be greater than any of its parts. This is something we know for certain. On the other hand, there is nothing in the natures of gentlemen and blondes that makes it necessary for gentlemen always to prefer blondes, and so this is only a matter of opinion.

The difference between knowledge and opinion can also be expressed in psychological terms. When we are asked, “Do gentlemen prefer blondes?” or “Will the Republicans win the 2000 election?”, we must make up our own mind. Nothing about the matter in question compels us to answer Yes or No. But when we are asked whether the whole is greater than any of its parts, we have no choice about the answer. If we put our mind to thinking about the relation of whole and part, we can think about the relation in only one way. The object we are thinking about makes up our mind for us.

This gives us a very clear criterion for telling whether what we assert is knowledge or opinion. It is knowledge when the object that we are thinking about compels us to think of it in a certain way. What we think then is not our personal opinion. But when the object of our thought leaves us free to make up our mind about it, one way or the other, then what we think is only an opinion — our personal opinion, voluntarily formed. Here other rational persons can differ with us.

On this understanding of the difference between knowledge and opinion, we must admit that most of our assertions are opinion. But we should also realize that opinions differ in their soundness. Some are based on considerable evidence or reasons which, while not conclusive, make them highly probable. Others are ill-founded, and others have no foundation at all but are simply willful prejudices on our part.

This leaves open the question whether history, mathematics, experimental science, and speculative philosophy should be classified as knowledge or opinion. As we have seen, the extreme skeptic would say that they are all opinion, though he might recognize that they have much more weight than mere personal opinions or private prejudices. The opposite view, which I would defend, is that we can have knowledge in the fields of mathematics and philosophy, and highly probable opinion in the fields of experimental science and history.

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