Grasping Reality on TypePad, by Brad DeLong

Tim Taylor: Some Thoughts About Economic Exposition in Math and Words: "[Paul Romer's] notion that math is 'both more precise and more opaque' than words is an insight worth keeping...

...It reminded me of an earlier set of rules from the great economist Alfred Marshall, who... wrote in 1906 letter to Alfred Bowley (reprinted in A.C. Pigou (ed.), Memorials of Alfred Marshall, 1925 edition, quotation on p. 427): 

But I know I had a growing feeling in the later years of my work at the subject that a good mathematical theorem dealing with economic hypotheses was very unlikely to be good economics; and I went more and more on the rules:

1. Use mathematics as a shorthand language; rather than as an engine of inquiry.

2. Keep to them till you have done.

3. Translate into English.

4. Then illustrate by examples that are important in real life.

5. Burn the mathematics.

6. If you can't succeed in 4, burn 3.

This last I did often.... Mathematics used in a Fellowship thesis by a man who is not a mathematician by nature—and I have come across a good deal of that—seems to me an unmixed evil. And I think you should do all you can to prevent people from using Mathematics in cases in which the English language is as short as the Mathematical...

Paul Romer adds some rules of his own...

1. The test is whether math adds to or detracts from clarity and precision. A writer can use either the words of everyday language or the symbols from math to make assertions that are clear and precise; or opaque and vague.
2. The deep problem is intent, not ability or skill.
3. Writers who want to make predictions use words and math to be clear and precise. Writers who want to make excuses use words and math to be opaque and vague.
4. Compared to words, math and code tend to be both more precise and more opaque...

I might quibble a bit with his rule #2, because my sense is that many of those who fail to communicate clearly, whether in math or in words, either have simply not put the time and effort into doing so, or lack the ability/skill to do so...

Dani Rodrik... includes the following...

1. Make your model simple enough to isolate specific causes and how they wor...
2. But not so simple that it leaves out key interactions among causes...
3. Unrealistic assumptions are OK...
4. Unrealistic critical assumptions are not OK...
5. Do not criticize an economist’s model because of its assumptions...
6. Ask how the results would change if certain problematic assumptions were more realistic...
7. Analysis requires simplicity...
8. Beware of incoherence that passes itself off as complexity...
9. Do not let math scare you...
10. Economists use math not because they are smart, but because they are not smart enough.

And, of course, John Maynard Keynes: John Maynard Keynes (1924): What Is the Excellence of an Economist?: "The study of economics does not seem to require...

...any specialised gifts of an unusually high order. Is it not, intellectually regarded, a very easy subject compared with the higher branches of philosophy and pure science? Yet good, or even competent, economists are the rarest of birds. An easy subject, at which very few excel! The paradox finds its explanation, perhaps, in that the master-economist must possess a rare combination of gifts. He must reach a high standard in several different directions and must combine talents not often found together. He must be mathematician, historian, statesman, philosopher—in some degree. He must understand symbols and speak in words. He must contemplate the particular in terms of the general, and touch abstract and concrete in the same flight of thought. He must study the present in the light of the past for the purposes of the future. No part of man's nature or his institutions must lie entirely outside his regard. He must be purposeful and disinterested in a simultaneous mood; as aloof and incorruptible as an artist, yet sometimes as near the earth as a politician...

• Tim Taylor (2018): Some Thoughts About Economic Exposition in Math and Words
• Dani Rodrik (2017): The Economics Debate, again and again
  • Larry Elliott (2017): Heretics welcome! Economics needs a new Reformation: "Neoclassical economics has become an unquestioned belief system and treats those challenging the creed as dangerous..."
• Brad DeLong (2016): What Is the Excellence of an Economist?
• Brad DeLong (2015): Richard Thaler Misbehaves–or, Rather, Behaves
• Paul Romer (2015): Mathiness in the Theory of Economic Growth
• John Maynard Keynes (1924): What Is the Excellence of an Economist?
• Alfred Marshall (1906): Letter to Alfred Bowley

• Brad DeLong: Optional Teaching Topic: How to Think Like an Economist... (Provided, That Is, You Wish to...) (Pre-Class? Mid-Class?)
  • Brad DeLong (2017): How to Think Like an Economist (If, That Is, You Wish to...)
  • Brad DeLong: How to Think Like an Economist—Lecture:
    • Expectational Weirdness: 1929: The Great Crash and the Great Depression
    • A Behavioral Relationship: The Production Function
    • An Equilibrium Condition: The Balanced-Growth Capital-Output Ratio
    • Thinking Like an Economist Slides
  • Brad DeLong (2016): A Few Scattered Notes, Observations, and Examples of Graphs and Diagrams in Teaching and Doing Economics:
    • Edgeworth Boxes and Production Possibility Frontiers
    • Paul Krugman (2016): In Defense of Funny Diagrams
    • J.M. Keynes (1924): Alfred Marshall, 1842-1924
    • John Maynard Keynes (1938): Letter to Roy Harrod: "Economics is a science of thinking in terms of models joined to the art of choosing models which are relevant to the contemporary world..."

• Brad DeLong (2015): Noah Smith, Paul Romer, "Mathiness", and Baking the Politics into the Microfoundations...:
  • Brad DeLong (2015): The Theory of Growth and Inequality: Piketty, Zucman, Krusell, Smith, and "Mathiness": It is Krusell and Smith (2014) that suffers from "mathiness"--people not in control of their models deploying algebra untethered to the real world in a manner that approaches gibberish...
  • Brad DeLong (2014): Department of "Huh?!"--I Don't Understand More and More of Piketty's Critics: Per Krusell and Tony Smith: As time passes, it seems to me that a larger and larger fraction of Piketty's critics are making arguments that really make no sense at all--that I really do not understand how people can believe them, or why anybody would think that anybody else would believe them. Today we have Per Krusell and Tony Smith assuming that the economy-wide capital depreciation rate δ is not 0.03 or 0.05 but 0.1--and it does make a huge difference...
  • Noah Smith (2015): Noahpinion: Paul Romer on mathiness
  • Brad DeLong (2015): Oddities in the Rhetoric of Economics: Paul Romer Confronts the "Adversarial Method" in the Presentation of Economic Theory
  • Paul Romer (2016): The Trouble with Macroeconomics

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