Can You Prove a Negative?
By Bruce Barbour - February 2024 (Version 1.0)
It is
sometimes argued that "you can't prove a negative". Another
closely related argument form is that "you can't prove a non
existence". Often this
argument is cited if an atheist is asked to prove that god
doesn't exist. (Often after the atheist asks the theist to
prove the existence of god.)
This
form of argument is a logical fallacy. That the argument
form is a fallacy is easy to show.
I will come back to the god example later. As a preliminary
example I make the claim to my colleague there is not a
six foot high, green, fire breathing, non magical dragon in the
next room. To prove it we only need to go into the next room and
after a quick (or slow and methodical) look around it will
indeed be apparent there is no six foot high, green, fire
breathing, non magical dragon in the room. Therefore the
negative or non existence - no dragon in the room - has been
proved. Easy.
So where did this confusion about the "impossibility" of proving
a negative come from?
Let's vary the example. This time my colleague make the simple
claim to me that there is not a dragon in the next room. I go
into the next room with my colleague. He says to me "see there
is no dragon in the room". But then I say to him "the dragon
could be invisible, that is why it wasn't found". Unconvinced we
bring in some equipment to the room that we think will enable an
invisible dragon to be detected. We still finds no evidence of
the dragon. But I am still not convinced. I say that, as well as
being invisible, the dragon could have magical powers and could
put up a veil of deception so my colleague cannot detect it. But
it could absolutely still be there. My colleague does not know
where to go from that. He cannot prove that there is not an
invisible magical dragon with perception masking abilities in
the room.
The difference between the first dragon claim and the second?
The first dragon was fully defined so what me and the colleague
were looking for was known and when it was not found we could
say definitively that the dragon did not exist. The second
dragon was not defined at all. Consequently we did not know how
to detect an undefined dragon.
What is the situation if the positive claim is made. That is (1)
there is a
six foot high, green, fire breathing, non magical dragon in
the next room. Or (2) there is a
dragon in the next room. The first option claim will
quickly be shown to be false by going into the room. The
second option claim will not be able to be shown to be
false - using a line of argument similar to the negative
claim.
Whether the claim trying to be proved is a positive or negative
claim has little impact on the difficulty of proof or falsity -
mostly. What has impact is how well defined what is being looked
for is. The challenge to "prove there is no god" is essentially
the same as the challenge to "prove there is a god".
There is one proviso. If you are looking to prove a positive
claim for something that is undefined, then even if you don't
know what you looking for you may still find it - though you may
have difficulty in recognising it. This could be through luck.
You are looking for something dragon like, you find something
and it happens to be what you are looking for. ("Look at what we
have stumbled upon. It's a strange phenomenon. That must be a
dragon.")
Another scenario is that what you are looking for is so all
pervasive that you could not but find it even though you weren't
sure of what you were looking for. ("My god. There's dragons
everywhere.")
Let's
look at the example of god. Can we prove that god doesn't exist?
It depends on how well we define god. If we said that god was an
old man sitting on a throne just above the clouds that may be
able to be disproved. The thousands of airplanes each day have
failed to see anything that resembles that definition of god.
For the purposes of this comparison only, god is like our
hypothesized undefined dragon. We don't know what we are looking
for. God is not sufficiently defined. And even if we can
eliminate any particular concept of god it does not eliminate
all the other possible concepts of god. And then there is the
problem that if god for what ever reason (unfathomable to us)
does not want to be found then we will not be able to find god.
The other aspect to keep in mind is, as
discussed here, the lack of evidence does not always
indicate evidence of lack. To say there is no evidence of, for
example, god therefore god does not exist is not logically
justified.
The
Need for Justification of all Knowledge
My stand
as an Agnostic
is that all Knowledge needs to be justified. Both:
- Positive
Knowledge – the Knowledge that something exists or that some
idea is true; and
- Negative
Knowledge – the Knowledge that something does not
exist or that an idea is false,
must be
justified for it to be classed as Knowledge. But sometimes this
is impossible due to lack of full definition of the thing or
idea being considered.
Summary
You may be
able to prove a negative claim provided it is well defined. You
can’t prove a negative that is not sufficiently defined.
You may be able to prove a positive claim provided it is
sufficiently defined. You may be able to prove a positive claim
that is not sufficiently defined – but this will be due to
either luck in finding what you were looking for, or else what
you were looking for was so readily apparent that it could not
be missed.
All Knowledge - positive or negative - needs to be justified.
The lack
of evidence for something is not necessarily the proof of
lack of existence.
Oversite Home Page.
|
