What is Scientific “Truth”…

… And How Do We Know We Found It?

by Brian D. McLean, BSc, DDS

What is Scientific "Truth" and How Do We Know We Found It?

What is scientific truth?  No one knows.

How do we know we found it? We never do.

These answers are blunt and short, and for most people I think, they are distressing.

Indeed, they are so distressing that most folks become dismissive and move on to something more palatable: “Nonsense! Of course we know some scientific truths!”

Without getting at the meaning driving the issue, the discussion can drift off into trivialities.

Surely there are some scientific truths;  there are twelve inches in a foot and a hundred centimetres in a metre. These facts are both true and scientific aren’t they?  They are scientific in the sense that scientists measure things and they are true by definition.

So what is meant by “we never know when we find a scientific truth?”

Science is about trying to understand and explain stuff about the universe.  The process involves identifying something we don’t understand (but would like to) and guessing what might be going on. The guess we come up with is called an hypothesis.  Let’s stick with the term guess.

The guess should be testable.  It should lead to predictions about other things: “If this is true then that will happen.” If “that” indeed happens, it proves that “this” is true.



Mankind has known that is wrong since at least the days of Aristotle, 2400 years ago. One of his thirteen logical fallacies is known as the error of affirming the consequent.

Say what?

In an “if-then” statement, the part before the “then” is known as the antecedent (comes before,) and that which follows the “then” is called the consequent (follows after.)

Easy. But why is it wrong?

Scientific “Truth” – a Daffy Example

Here’s an observation I have made about the world:

“Whenever it rains, the streets get wet.”

That leads me to guess:

“If  the streets are wet, then it is (or has recently been) raining.”

That is such a shatteringly brilliant insight, I’m tempted to use the more scholarly and impressive term “hypothesis.” But I said we would stick with the term “guess.”  What a pity!

Now, the guess can be tested by frequently checking the condition of the streets and the associated weather. Time after time, whenever the streets are wet (the antecedent), everyone affirms that it is or has been raining (the consequent part of my guess).

Hurray! My brilliant hypothesis has been proved! No longer must mankind wallow in superstition surrounding the age-old mystery of wet pavement! I withdraw my pledge to stick with “guess.”  This is just too important a discovery to debase with such unsophisticated language. Perhaps it will become known as McLean’s Law.

I’m just bursting with pride about the new scientific law I have discovered. Surely, you too, are as impressed with me as I am.

Then,  one sunny day, my friend – well, he used to be my friend – reports that the streets have become wet despite the lack of rain.  The sewers just backed up. Really crappy evidence, I call it.

I still maintain, of course, that “the great weight of evidence” supports the well-accepted truth of McLean’s Law. I am indignant. Obviously this bogus, new, so-called “evidence” should be ignored because it is not consistent with what we already know. There comes a point, doesn’t there, when experts (like me) should exercise our right to scoff at, or simply ignore evidence advanced by newcomers (like my former friend)? We experts do this for the good of all. We want to protect the lay public from being distracted by silly, new “fringe” ideas. We want to keep them focused on the truth.

Despite my protests, eventually McLean’s Law is going into the trash heap where it belongs.  However, we authorities have learned many skills to delay such disasters. It can take ages to reach the trash.  Intellectual inertia, especially when it enjoys the status of institutional policy, is stunningly effective.

“Proven” Science Doesn’t Exist

While the fantasy of my scientific glory is silly, it makes an important point or two. An hypothesis cannot be proven to be true by the only means available to us – checking the consequences to see if what was predicted actually happens.  It can be shown apparently to be true by many examples.  It can become believed to be true by the repetitive confirmation that its predictions are correct (a.k.a. “affirming the consequent.”)  It will probably become “accepted” as true.  Some people will commit the unpardonable sin of calling it a “scientific fact.” Don’t you remember those glorious days when we all believed in McLean’s Law?

What brought about the demise of McLean’s Law? My so-called friend stumbled upon an instance of wet streets  without a history of rain.  So much for “If  the streets are wet, then it is (or has recently been) raining.”

That unspeakable rascal disproved McLean’s Law by the simple, logically valid expedient that Aristotle called “denying the consequent.”  Now, they say, no longer hobbled by a belief in a bad guess, – (I thought it was a law!) – we are free to make better, more enlightened guesses about how the streets get wet, and even guesses about useful things that we actually want to try to understand.

Things like, “If  we lower cholesterol levels with drugs, then we will prevent heart attacks and not create other illness”  or “if we add  fluoride to community drinking water then we will prevent cavities without creating other health problems,” or “if we fix damaged teeth with metals dissolved in mercury then we will not threaten one’s health in other ways.”

The medical and dental literature is indeed full of instances that affirm the consequent of the above guesses. Its experts tell us that this proves that the guesses are true.  In so doing, they are claiming that “affirming the consequent” is logically valid.  But it is not.  They claim to possess scientific truth. There is no logical justification whatsoever to make that claim. Even if they actually had “truth,” they would have no way of knowing it.

To summarize: It is logically invalid to affirm the consequent because one never knows if or when evidence will appear that could not possibly exist if the guess were correct. It is logically valid to deny the consequent.  If the falsifying evidence is discovered, then there it is.

In fact, all the scientific stuff mankind believes to be true, it believes through the false logic of affirming the consequent. Some of it, we earnestly hope, is  “truth.”  Some of it is not.  Since human beings lack the ability to determine whether guesses are true, we simply “accept” them as true.  We’re stuck with that.  It’s the best we can do.

A Passion for Valid Logic

Well, actually, it’s the best we can do…

     except for





Realizing that all the scientific “truth” we possess is tentative and very likely flawed or often downright wrong, we ought to be struggling constantly to utilize the logically valid process of “denying the consequent.”

We want to trash more McLean’s Laws.

And when McLean claims that “the great weight of evidence” trumps the one instance of verified evidence that trashes his law, ignore him.

Just as McLean (me) sulks at the demise of my “law”, so did Thomas Huxley, the nineteenth-century progenitor of generations of illustrious Huxleys declare with mock outrage:

“The great tragedy of science – a beautiful hypothesis slain by an ugly fact.”

Although McLean (me) feels rejected and inclined to whine about the injustice done me,  really great scientists (sigh) have a different attitude:

With respect to affirming/denying the consequent, Albert Einstein said:

“No amount of experimentation can ever prove me right; a single experiment can prove me wrong.”

With respect to being wrong and what to do about it, Einstein said:

“My ideas caused people to reexamine Newtonian physics. It is inevitable that my own ideas will be reexamined and supplanted.  If they are not, there will have been a gross failure somewhere.”

When we find a scientific guess to be wrong, the task becomes one of making a new guess; one that seems to explain all the observations and phenomena that the old one did and attempts to explain the pesky new phenomenon, the “ugly fact,”  that caused the demise of the old guess.  That new guess will hopefully be a better one since it accounts for all the pertinent observations.

Then we will know the truth, right?

By now you know the answer is – Wrong!

Then will follow lots of experiments that “support the hypothesis” thus appearing to show that it is correct. This is more “affirming the consequent.” We come to believe the new guess. But, this time, let us not commit to the new belief with passion. We have no right to do so.

From now on, let our passion be reserved for relentlessly and zealously seeking out “ugly” facts that show that what we think is right is demonstrably wrong. When the experts imply that they possess the scientific truth, have some passion to discover if there is already published falsifying evidence which those experts are choosing to ignore. The experts may think such evidence is ugly. Such experts are wrong. “Ugly facts” allow science to progress by the process of “myth-busting.”


“A foolish faith in authority is the worst enemy of truth.” – Albert Einstein


And please save some passion for Mark Twain:

“What gets us into trouble is not what we don’t know, it’s what we know for sure that just ain’t so.”

And reserve yet more passion for Nobel Laureate Richard Feynman who explained that “The Key to Science” is “If it disagrees with experiment, it’s wrong. That’s all there is to it.”

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