Sometimes arguments are about real stuff, and sometimes they merely reflect confusion as to the meaning of the words and terminology people use in discussions and debates.  In the latter case, underlying assumptions are rarely stated, and each participant will, according to human nature, assign their own meaning, leading to sometimes acrimonious and sometimes comical misunderstanding.  This phenomenon is aptly captured in one of my all-time favorite aphorisms, quoted above.

Pro cameras -- what's in a name?

Take the case of the endless debate about what constitutes a professional camera.  At one end of the spectrum there is the very expensive line of ruggedized and weather-resistant, high performance, full-featured, integrated vertical grip cameras that many professional sports and action photographers use.  As of this writing (Summer 2019), such products include Canon's 1DX MkII and Nikon's D5.  Sony's A9 fits in this category, albeit with a less robust build and without the integrated vertical grip.  No one would question that these are "professional" cameras -- although many well-heeled amateurs happily use them in a variety of applications as well.

But what about somewhat less expensive models, such as Canon's 5D MkIV or Nikon's D850?  Or Sony's multifaceted A7 line?  These cameras often have higher resolution but lesser frame rates and are a bit less rugged than the above.  Nevertheless, they still provide good performance and an extensive feature set, and their lower cost and great flexibility makes them favorites with pros for a variety of applications, including weddings, portraits, studio setups, photojournalism and event photography, among others.  Are they not "professional" cameras as well?

Historically, there is precedent.  Early top-of-the-line full frame high frame rate and high resolution cameras started out as parallel fully ruggedized "pro" bodies sharing many of the same build attributes.  But, makers eventually realized that the needs of the latter category of photographers could be met with a somewhat less rugged -- and less expensive -- body.  Even so, there is certainly overlap in function and utility.

Stopped clock, what time is it?

At the other end of the spectrum, there are purists who proclaim that "any camera a professional photographer uses is a professional camera". Unfortunately, that statement can come across as shallow Internet frivolity.  By that standard, every camera potentially becomes a professional camera, no matter how mundane or ill-suited to the rigors and demands of normal professional use.   For such pronouncements, the term ceases to have significance.  And, worse, it sidetracks meaningful discussion.  After all, people once created images with pinhole cameras and camera obscuras. In fact, in their day, they might have been the only tool available to professionals.  But time and the technology moves on.  Try using one of those at the next Olympics!

This blanket claim resembles the observation that a stopped clock is correct twice a day -- leaving one to puzzle over exactly which two moments in time the clock might be correct.  The folly of either assertion can easily be understood by appeal to information theory, which states that the information, I, conveyed by any proposition, x, is defined as the logarithm of the inverse of the probability of its occurrence.¹

I_x = log₂ (1/p_x)

In the case of the stopped clock, the probability of the clock reading the stopped value is p = 1, no matter what time it actually is.  The inverse is 1/1 = 1.  And, the logarithm of 1 is zero.  Thus, I_{stopped clock} = log₂ (1/1) = 0.  Which demonstrates mathematically what common sense told us all along:  that a stopped clock conveys exactly zero information about what time it is at any particular moment during the day -- the latter being the actual purpose of a clock.

Hopefully, it isn't too much of a stretch of the imagination, even on the part of today's Internet-addled generation, to see that it also provides no useful information to state that every camera that has ever been used by a professional is a "professional" camera, thus potentially encompassing every camera ever made.  Under that definition, the probability of any camera being a pro camera is a 100% certainty.  Therefore, as with the stopped clock, the information transmitted is zero.  That's null-point-naught to the gazillionth decimal place for those who require really big numbers to catch their attention. 

Will the real pro camera please stand up

This, hopefully, leads to a more realistic discussion of what does and doesn't constitute a professional camera.  It seems that the category is defined by capability, suitability and useage.  Of course, there's always the opinion of experienced commentators, for instance the assessment of Nikon expert Thom Hogan regarding current generation mirrorless models -- note that his qualifying gear decidedly does not include every camera ever made. ²  A more systematic approach might be to exhaustively examine the universe of cameras used by professionals and build a histogram of the various product lines routinely used for paid work.  Histograms, for the uninitiated, are stand-pipe charts where the height of a particular bar represents the number (frequency) of occurrences of that particular entity.  From Wikipedia, for example:

While it is undoubtedly a bridge too far to gather data on all cameras ever used by professional photographers for business purposes, one can make some reasonable suppositions that should get us in the ballpark.  First, those high frame rate, ruggedized bricks one sees on the sidelines of sporting events will certainly have a visible presence, either in absolute terms or as a percentage of the total among sports and action photographers.

And, the next line down in expense and capability, the favorites of wedding, portrait, studio, and event photographers, will probably be even more numerous.  There may, of course, be a smattering of other product lines, but it is likely that most of their frequency bars will be significantly shorter than the two categories above.

Note that we are counting together all brands with similar capabilities as well as entire product lines across multiple generations.  Thus, using Canon's line for instance, one may find 1DX, 1DX MkII, 1D MkIV, etc., as well as many generations of comparable models from other manufacturers, all in use by professional sports photographers.

Also, it may be necessary to consider different types of subject matter separately to get an accurate picture, since some photographic specialties may be disproportionately represented compared to others.  For instance, are there more sports photographers than wedding photographers -- or vice versa?  The US Bureau of Labor statistics provides little help in this task, lumping most professional photographers into a catch-all category labeled "Other Professional, Scientific, and Technical Services." ³

Making the terminology clear

Because there appears to be at least two (and possibly more) product lines that are frequently employed by professionals, one must qualify the term with a modifier that identifies which group one is talking about.  There are a myriad of ways one can go with this, and about the best one can do is pick widely understood descriptive terms.  For instance, one might describe those ruggedized high frame rate cameras used by many sports photographers as professional action cameras -- realizing of course that these cameras can be, and are, used for many other applications.  The next line might be described as general purpose professional cameras, realizing also that for some applications these (and other) types may be interchangeable.

Whatever the outcome, it would seem that a working definition of a professional camera is a feature and performance appropriate imaging device built with sufficient robustness and reliability to enable professional photographers to perform their jobs effectively, with a minimum of compromise due to the limitations of their gear, when used for making images in their particular field of interest.  Based on real world use instances, some cameras will predominate as workhorses and others will find substantially fewer takers.

¹ https://brilliant.org/wiki/entropy-information-theory/

² http://www.sansmirror.com/newsviews/2019-mirrorless-camera/april-june-2019-mirrorless/what-constitutes-pro.html

³ https://www.bls.gov/oes/current/oes274021.htm