The lines of the following poem are phrases selected from consecutive pages of the second chapter of Paul Dirac’s Principles of Quantum Mechanics, Fourth Edition (Revised), *Dynamical Variables and Observables*.

**we may look upon the passage**

for the triple product

We therefore make the general rule

in spite of this fundamental difference

which conforms with our notation

The rule may easily be extended

By repeating the process

Using this terminology, we can assert that

Suppose we have a solution

Taking the conjugate imaginary

which contradicts the assumption

It is obvious physically

Not every real dynamical variable can be measured

if the second measurement is not actually made

Let us examine mathematically

even though it is beyond the power of present-day mathematical analysis

The form of infinity required for this

is called by mathematicians

a more general definition of a function

which we have seen is impossible

we are able to give a meaning

when it exists

If we took different signs

neither of which is connected in any simple way

We can go further and speak

of what can be attained in practice

This suggests that the chances for the existence

will be generally followed in the future

generalize the results

This conclusion is an important new development

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I like how tired he sounds. It‘s almost like I hear his voice, I can imagine the inflection so well.

Absolutely true! I also find it amazing when I read the book. It’s on the one hand such a polished and perfect treatise … but still you hear a unique voice. But I read in a biography that Dirac gave his lectures the way he wrote his papers.