
Ask HN: Can a tautology be serious? - bruno_rzn
Let&#x27;s write a tautology : A = A
Let&#x27;s rewrite it : A = A&#x2F;B * B<p>Now, I often see people (in economics, mostly) do this little trick :<p>ΔA = Δ(A&#x2F;B * B)
Δ * ln(A) = Δ * ln(A&#x2F;B) + Δ * ln(B)<p>Then, they will show that, empirically, A&#x2F;B has always been constant (more or less), thus, if you want to increase A, you just have to work on increasing B.<p>Can this kind of equation actually say something about reality ? 
Can it be falsified, so it can be taken seriously to be used for predictions ?<p>(to add some context, I&#x27;m thinking here about Kaya identity)
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nanis
The Kaya identity (just like the GDP accounting identity abused by all sorts
of interventionists) is devoid of empirical content.

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bruno_rzn
Could you generalize to other identities ?

Or, on the contrary, give an example of such equation with some actual
empirical value ?

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nanis
Well, at least the GDP identity is an accounting identity, so it has some
value, but the causal relationships are not there.

Every mathematical statement is a tautology (in that they are inevitable
conclusions logically derived from a set of axioms). If the axioms constitute
a decent abstraction of reality, then you can try to test causal relationships
uncovered by logic, or use them in practical ways (Euclidean geometry,
Newton's laws etc).

You can read Kaya's identity as per capita CO2 emissions from human sources is
equal to per capita GDP times energy use per GDP unit times CO2 emissions per
energy unit production. So, you have three ways of reducing per capita CO2
emissions if you are a global central planner (pushing aside the miserable
implications of having a global central planner): 1) reduce living standards;
2) use energy more efficiently; and 3) generate less CO2 when producing
energy.

And, if all you care about is reducing total CO2 emissions, start culling
people or reduce the birth rate. This appeals to the 1970s neo-Malthusian,
"the world is going to run out of food" crowd cause they make a living selling
catastrophe. Combined with neo-Marxists who like to emphasize distribution
much more than the absolute level of wealth, they like to ignore how living
standards have improved[1] contemporaneously with increased CO2 emissions.

[1]: [https://www.vox.com/the-big-
idea/2016/12/23/14062168/history...](https://www.vox.com/the-big-
idea/2016/12/23/14062168/history-global-conditions-charts-life-span-poverty)

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bruno_rzn
Thank you again for this answer.

What I really don't understand here is the transition between :

1 - You can read Kaya's identity as per capita CO2 emissions from human
sources is equal to per capita GDP times energy use per GDP unit times CO2
emissions per energy unit production.

and

2 - So, you have three ways of reducing per capita CO2 emissions...

What is the logic behind this ? I mean, if you pose [ CO2 = CO2/pop * pop ]
(simplified, but indeed always true by definition), how can you say that one
way of reducing the CO2 emissions is reducing the [pop] term ?

I don't know if I'm clear enough...

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nanis
> What is the logic behind this?

I guess my disdain for this kind of approach was not obvious.

