In recent years it has become evident that the primary concept of the photon has multiple interpretations, with widely
differing secondary connotations. Despite the all-pervasive nature of this concept in science, some of the ancillary
properties with which the photon is attributed in certain areas of application sit uneasily alongside those invoked in other
areas. Certainly the range of applications extends far beyond what was envisaged in the original conception, now
entering subjects extending from elementary particle physics and cosmology through to spectroscopy, statistical
mechanics and photochemistry. Addressing this diverse context invites the question: What is there, that it is possible to
assert as incontrovertibly true about the photon? Which properties are non-controversial, if others are the subject of
debate? This paper describes an attempt to answer these questions, establishing as far as possible an irreducible core of
what can rightly be asserted about the photon, and setting aside some of what often is, but should never be so asserted.
Some of the more bewildering difficulties and differences of interpretation owe their origin to careless descriptions,
highlighting a need to guard semantic precision; although simplifications are frequently and naturally expedient for
didactic purposes, they carry the risk of becoming indelible. Focusing on such issues, the aim is to identify how much or
how little about the photon can be regarded as truly non-controversial.