Note: This is part four in a multi-part series. Part one can be read here, part two here and part three here.

In the aftermath of every mass shooting to make national headlines (Orlando would be the most recent and arguably biggest), cries of “gun control” and “no gun control” flood the internet and the airwaves. Somewhere in both arguments is the idea of 'common sense gun laws.'

President Obama is a fan of this phrase. He's used it in remarks made in January and a town hall meeting earlier this month, among various other speeches made throughout his presidency.

There's just one thing: the term 'common sense' differs from person to person on guns. For instance, what Conan O'Brien, Cher or Samantha Bee (warning: language) feel is common sense differs largely from what Vince Vaughn, Brad Pitt (and wife Angelina Jolie) or Joe Mantegna think. In other words, there's no real definition here.

In a day and age where gun sales are at an all time high and more states are allowing for weapons carry, suggested forms of regulation (or non-regulation) comprise as wide a spectrum as their supporters.

One idea being tossed around is that of the Department of Homeland Security's No Fly List, mentioned by Obama in the town hall meeting. It's decent logic: if you're suspected of terrorism or associate with known terrorists, not only can you not get on a flight, you shouldn't be allowed to buy a firearm.

However, as CNN, The Boston Globe, the ACLU, Congressman Trey Gowdy (R-S.C.) and others have pointed out, the list has its flaws. It's estimated that over 47,000 names are comprise the list which has historically included the late Sen. Ted Kennedy (D- Mass.), singer Yusuf Islam (formerly Cat Stevens) and various children under the age of 10. Even known Federal Air Marshals have somehow gotten flagged before a flight.

And if you've landed on the list, it's harder to get off it, as Rahinah Ibrahim found out -- after being flagged in 2005, it took her a good nine years to successfully sue the government. The major issue was that she never found out why she was on it, to begin with, something Gowdy spoke of -- there is no due process of law afforded by the FBI regarding the Terrorist Watchlist as a whole, of which the No Fly List is just a mere fraction. The lack of transparency means that there are no specific criteria for getting on or off of either list, similar to Sen. Joe McCarthy's (R- Wis.) list of accused Communists within the United States government in the 1950s.

Another idea often brought up is that of mental health. Representatives of the Republican Party, like Sen. Marco Rubio (R-Fla.) and former Governor Mike Huckabee (R-Ark.), tend to place the blame of mass shootings involving lone gunmen on mental illness, and there's some degree of fact behind this line of thought. Aurora, CO shooter James Holmes was found to be mentally ill during court proceedings, while Newtown, CT shooter Adam Lanza had previously been treated at Yale University for autism and antisocial behavior.

Now, Rubio has worked in the past to introduce legislation for mental health services alongside Sen. Ted Cruz (R-TX) and a bipartisan group (it failed), but he's one of the few exceptions. Sen. Rand Paul (R- Ky.), former Governor Jeb Bush (R- Fl.) and Huckabee have all spoken out on mental health but rarely passed legislation towards promoting it. In fact, Arkansas earned a D grade from the National Association on Mental Illness during Huckabee's tenure, as did Florida under Bush's governorship.

Although these issues are important to discuss and there are mass shooters out there who have mental issues, it's still found that the mentally ill are not the norm for gun violence toward others but the odds of self harm in comparison are drastically higher, which would lead to a discussion on the use of guns in suicides. In addition, not all mass shooters are found to be mentally incompetent, either -- it's generally understood that Charleston, SC shooter Dylann Roof was racially motivated in his attack, for example.

The hard part about the term 'common sense' is that it implies that the overwhelming majority of a population would agree on it. Common sense says that sticking your hand in the fire will burn you or that not paying your electric bill gets your lights shut off. Unfortunately, it says nothing about how to handle gun violence when it arises in our world.