The lines of the following poem are phrases selected from consecutive pages of the first chapter of Paul Dirac’s Principles of Quantum Mechanics, Fourth Edition (Revised), The Principle of Superposition.

~

one would be inclined to think
There must certainly be some internal motion

from general philosophical grounds
we cannot expect to find any causal connexion

observe what appears on the back side of the crystal
all that can legitimately be asked

somewhere in the region of space through which the beam is passing
the photon must change suddenly

Each photon then interferes only with itself
The reader may possibly feel dissatisfied

make precise the important concept of a ‘state’
there exist peculiar relationships

such a drastic departure from ordinary ideas
analogies are thus liable to be misleading

We shall begin to setup the scheme
We shall call them ket vectors

any length one may assign to the ket vector is irrelevant
we can always set up a second set of vectors

One may look upon the symbols < and >
call one of them the conjugate imaginary of the other

There is thus a kind of perpendicularity in these spaces
The relations appear in mathematical form, but they imply physical conditions