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Can Reason Err? Searching for

Common Ground between Naturalists and Theists

No useful discussion is possible unless both parties to the discussion start from the same premise.

MediŠval Maxim


1) The word “prove” has two distinct meanings, one strict and one colloquial, that have very little to do with one another. The strict meaning of “prove,” as understood by logicians and mathematicians, is to reason syllogistically such that the conclusion states explicitly what was implicit in the premises. The ordinary wide meaning of “prove” is to ascertain knowledge with a high degree of probability through evidence and reason—as in the phrase “innocent until proven guilty.”

2) In the main, “reason” is about making rational inferences from “reasonable” premises. But most of the rational inferences we make in philosophy and science, as well as in practical life, are non-demonstrative. In other words, most of the inferences employed by reason are not, strictly speaking, logical (i.e. demonstrative or deductive) inferences.

3) Inductive inferences are not only not logical, they are indefensible in logic. Nevertheless they qualify as rational inferences—provided they are governed by the dictates of the mysterious thing called “common sense.” It is common sense that tells us that inductive inferences are rational when a) they give us a feeling of a high degree of certainty, and/or b) we can’t do without them.

4) Philosophical reasoning must not be divorced from the common sense that presides over our daily behavior. Thus, there are some inferences which are neither deductive nor inductive, but are universally accepted as common sense—such as the belief that other people have minds; the belief that things (like a house or a mountain) continue to exist when no one is looking at them; the belief that when you “perceive” a table or a person it is because what you are perceiving is there to be perceived; the belief that we live in a common world, peopled not only by sentient beings like ourselves, but also by physical objects; etc., etc. These common sense inferences are to be taken as rational until we have good grounds for thinking otherwise.

5) For philosophical purposes, there is no dichotomy between knowledge and belief. Whatever we know, we also believe. “Knowledge” is the word we reserve for beliefs that are founded on evidence and reason and that give us a feeling of a high degree of certainty. Knowledge is also a matter of degree and shades into rational faith, that is, shades into belief for which we have some, but not conclusive, evidence.

6) For philosophical purposes, there is no sharp frontier between reason and some kinds of feeling, even when “feeling” is not being used in the sense of “perception”—unlike Bertrand Russell’s sentence, ‘There are philosophers who hold that all reality is mental, and that while the feelings [my italics] we experience when we look at the sun are real, the sun itself is a fiction.’ There are many feelings which are undeniably emotional, such as affection or grief, that are universally recognized as reasonable. Indeed many concepts, such as justice and injustice, kindness and cruelty, etc. depend on a mixture of reason and emotion.

7) Strictly speaking, facts never speak for themselves. Facts only become “evidence” after they have been interpreted.

8) What we learn from experience depends on the kind of philosophy (or overarching interpretation) we bring to experience.

9) Similarly, how we weigh evidence depends on the kind of philosophy we bring to the evidence.

10) Whether the principles by which we interpret our experience (or the facts) are rational or irrational is often a matter of philosophical opinion. In other words, there is no objective criterion or standard by which certain competing principles of interpretation can be judged. The only thing we can do is appeal to common sense, despite being only too well aware that we seldom attribute common sense except to those who agree with us.


The word ‘proof’ is not ordinarily restricted in its application to demonstratively valid arguments, that is, [arguments] in which the conclusion cannot be denied without thereby contradicting the premises. (1)

Anthony Flew

If nothing is self-evident, nothing can be proved. There are some premises that can’t be reached as conclusions. (1)

C. S. Lewis

You can only find truth with logic if you have already found truth without it. (1)

G. K. Chesterton

[In 1944 at Trinity College, Oxford] I chose as the subject of my annual course, ‘Non-Demonstrative Inference.’ I had become increasingly aware of the very limited scope of deductive inference as practised in logic and pure mathematics. I realized that all the inferences used both in common sense and in science are of a different sort from those in deductive logic, and are such that, when the premises are true and the reasoning correct, the conclusion is only probable. (2)

Bertrand Russell

It was Hume who first pointed out, and with all his customary lucidity, that from no finite number of observations, however large, could any unrestrictedly general conclusion be drawn that would be defensible in logic. If every time I let go of something it falls, I might well conclude eventually that all unsupported objects fall. But the conclusion has been reached from the premises not by a logical process but by a psychological one. (3)

Bryan Magee

We really don’t have any choice but to assume continued regularity. This is what science does, and so far so good. (3)


Reason is to be defined as “instinct enlightened by reflection,” and the primitive beliefs that guide our conduct are instinctive in nature. (4)

Newton P. Stallknecht

Instinct, intuition, or insight is what first leads to the beliefs which subsequent reason confirms or confutes; but the confirmation, where it is possible, consists, in the last analysis, of agreement with other beliefs no less instinctive. Reason is a harmonizing, controlling force rather than a creative one. Even in the most purely logical realms, it is insight that first arrives at what is new. (4)

Bertrand Russell

Logical certainty may be absent, but reason, which requires empirical evidence, does not always demand logical certainty. In the case of the cup on the table that we all see, it is reasonable to say we are absolutely certain that it exists. (4)

Many probabilities seem to us so strong that the absence of logical certainty doesn’t induce in us the least shade of doubt. (4)

C. S. Lewis

“It may be true in theory, but it does not work out in practice,” is a favourite maxim of the plain, blunt man. He does not realise that he is merely echoing the ancient heresy of Averroes. A thing cannot be true in theory and yet false in practice. Practice is the empirical check on theory, and if practice confutes theory, theory must be revised. (4)

Arnold Lunn

There is no such thing as a philosophy which does not receive its impulse and impetus from a prior and uncritically accepted interpretation of the world as a whole. (4)

Josef Pieper

I do not think it is possible to get anywhere if we start from scepticism. We must start from a broad acceptance of whatever seems to be knowledge and is not rejected for some specific reason. Hypothetical scepticism is useful in logical dissection. It enables us to see how far we can get without this or that premise—as, for example, we can inquire how much of geometry is possible without the axiom of parallels. But it is only for such purposes that hypothetical scepticism is useful. (4)

Bertrand Russell

He who wishes to learn must believe. (5)


If we have better grounds for believing something to be true than for believing it not to be true, it is not irrational to invest a certain degree of faith in it. There is nothing unscientific about such an attitude because the question at issue, concerning as it does the untestable, is not a scientific question. (5)

Bryan Magee

The question ‘what do we mean by “knowledge”?’ is not one to which there is a definite and unambiguous answer, any more than to the question ‘what do we mean by “baldness”?’ (5)

Bertrand Russell

[We often hear] about the necessity of keeping all emotion out of our intellectual processes—“you can’t think straight unless you are cool.” But then neither can you think deep if you are. I suppose one must try every problem in both states. You remember that the ancient Persians debated everything twice: once when they were drunk and once when they were sober. (6)

C. S. Lewis

Facts are often part of a web of evidence—facts may or may not be evidence, depending on the context—but the same set of agreed facts can be taken as evidence in support of different interpretations or theories of the overall situation. (7)


I don’t think [the fine tuning argument] proves anything, but it is entirely reasonable for people who already have a belief in a creating God to regard this as confirming evidence. It’s a point of argument which I think very important: to see that what is reasonable for people to do in the face of new evidence depends on what they previously had good reason to believe. (9)

Anthony Flew (since converting to deism in 2004)

Philosophical argument, strictly speaking, consists mainly of an endeavour to cause the reader to perceive what has been perceived by the author. The argument, in short, is not of the nature of proof, but of exhortation: “Look, can’t you see what I see?” (10)

Alan Wood


Upon the whole, then, it appears, that no testimony for any kind of miracle has ever amounted to a probability, much less to a proof; and that, even supposing it amounted to a proof, it would be opposed by another proof; derived from the very nature of the fact, which it would endeavour to establish. It is experience only, which gives authority to human testimony; and it is the same experience, which assures us of the laws of nature. (1)

David Hume

I have learned that arguments, no matter how watertight, often fall on deaf ears. I am myself the author of arguments that I consider rigorous and unanswerable but that are often not so much rebutted or even dismissed as simply ignored. (1)

Daniel Dennett

I am prepared to admit the possibility that I am nothing but a biologically and socially convenient fiction, that some hundreds of millions of Buddhists, in fact, are correct in referring to “the illusion of personal identity.” (4)

J. B. S. Haldane

The arguments of the sceptic are, says Hume, valid. But only theoretically. Having conceded their validity as arguments he drives home the point that it is impossible for anyone actually to live as a sceptic. (4)

Bryan Magee

Long ago I set about systematically changing the experience [of acting freely]. I now have no feeling of acting with free will, although the feeling took many years to ebb away. . . As for giving up the sense of an inner conscious self altogether—this is very much harder. I just keep on seeming to exist. But though I cannot prove it, I think it is true that I don’t. (1, 4, 8)

Susan Blackmore

What, after all, is faith? It is a state of mind that leads people to believe something—it doesn’t matter what—in the total absence of supporting evidence. If there were good supporting evidence then faith would be superfluous, for the evidence would compel us to believe it anyway. (5)

Richard Dawkins

It is wrong always, everywhere, and for everyone, to believe anything on insufficient evidence. (9)

W. K. Clifford

Science, in the broadest sense, includes all reasonable claims to knowledge about ourselves and the world. (10)

Sam Harris

No testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous than the fact which it endeavours to establish. (10)

David Hume

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