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