I feel pain, therefore I exist?

Years ago, one video that I used in my classes was about the economic geography of Gloucester, Massachusetts.  The city, the entire cape, has a long and rich fishing heritage.  But, it is also a textbook example of overfishing, which then leads to an economic decline in an area that hadn't diversified from what was once a booming industry.

In one scene in the video, a fisher stamps his foot over a iced container of fish.  Even the students who love to fish almost involuntarily reacted in horror, which always interested me.  

I don't eat fish--both because of the alien smell and taste, and because I have seen one too many movies and television shows in which the baited fish struggles to survive out of water.  But, to the students, the smell and taste of fish is a part of their life about which they don't think, and most of them have their own personal fishing stories too, which often involves their grandfathers.  Yet, they recoiled more than I did when the fisher in the video stomps in the vat of ice and fish.

During one of those class discussions, it was clear that students did not want to talk about their feelings about fish being harvested and iced and cooked.  It was almost a "can we not talk about that, please!" situation.  My class was not about feelings, and I didn't see any reason to pursue that issue.

To an orthodox Jain, this issue doesn't arise.  They even refuse to eat vegetables that grow under the soil out of a concern that harvesting them could also kill the tiny life forms around them.  Further, something like a potato is "alive"--it can sprout and propagate.

Most of us aren't that strict.  What we instead do, explicitly or implicitly, is to draw a line that clarifies when it is acceptable to kill an animal.  Whether it is for consumption, like in the case of lobsters, or at our homes when we slam the life out of spiders, we make our own rules.  Beef-eaters think that people who eat dog meat or beetles are crazies, while people who worship cows think that the beef eaters are crazies.  And, some of us apologize to the critter for killing them!  

In a secular context, we even offer arguments that a spider or a lobster doesn't really feel pain.  Those are not highly developed animals capable of such feelings, we claim.

But, what if even worms feel pain?

The documentary My Octopus Teacher gave us watchers plenty of video footage to think about the octopus.  When an octopus shows that much intelligence and emotion, we can certainly imagine that they feel pain, right?  Should we recognize these animals as sentient?

Is it ok to kill the fly that has been bugging me?