Update (February 11, 2020):
I just learned that sponges are considered animals even though they do not move. Because of that, my definition for animal below falls apart. (Please do not read this post. You will just be wasting your time if you do.)
Also C.S. Lewis, a new addition to my list of heroes, referred to people as animals in his book Mere Christianity.
I can now somewhat accept that people be classified as animals, with a strong note of course that people are not just animals. We might be classified as animals but being an animal is just a subset of what we are.
(Below is the original content of this post)
I was taught, when I was a child, that people are special beings. That they are different from animals. By animals I mean cats, dogs, cows… You know what I mean. 
Why would some professors at MIT categorize people as animals?
For instance, at about 21:00 minutes in the video of Lecture 9 of MIT OCW’s course #6.0001, the lecturer, a beautiful and intelligent lady, categorizes people as “animals”.
(I am learning a lot from these lectures. But this “animal” thing is not CS-related, so I believe I am allowed to comment on this. I might sound stupid below, but bear with me till the end.)
Ahh! Maybe they have a different definition for “animal” than I do.
Maybe their definition for “animal” is
“any living thing that is not a plant“.
Maybe not… A bacteria will be considered as an animal by that definition.
Maybe their definition for animal is
“any living thing that is not a plant and is not a bacteria“.
Are viruses different from bacteria?
I don’t know. But if they are, I have to change the definition… An animal is
“any living thing that is not a plant and is not a bacteria and is not a virus“.
Are insects animals too? I’m not sure. If they are not, then I have to modify the definition a bit… An animal is
“any living thing that is not a plant and is not a bacteria and is not a virus and is not an insect“.
Are angels animals also?
Wait, wait, wait! Are angels even allowed in this discussion?
Okay, let’s exclude angels in this discussion.
But if we don’t consider bacteria and viruses as animals because they are very different from what we know as animals (like cats, dogs, and rabbits) why should we classify ourselves (people) as animals when we know that we are very different from cats and dogs and rabbits?
We know?
Hmmm…
Wait…
Perhaps we can make our definition much simpler, to make the difference between us and bacteria very clear.
Okay, let’s start from the beginning…
What about we define “animal” as
“anything that has life”.
That will make our lives easier. Right?
But plants are clearly not animals! They are very different from what we think of as animals!
Clearly? Clear is very subjective. What is clear to you might not be clear to others. What’s clear today might not be clear tomorrow!
But… for now, we will just follow what we think is clear today.
Okay… An animal is
“any living thing that is not a plant“.
(What is a plant?)
That should be our last attempt of defining “animal”, right? — Perhaps we can convince everyone that bacteria are animals also!? They are very tiny, but they can move, like cats can move!
But when we hear the word “animal”, what we are thinking of is something that can be seen by our naked eye!
Bacteria cannot be seen with the naked eye! Viruses too!
… Ahh!… I think I get it already.
Perhaps the definition for “animal” is
any living thing that can move and can be seen with the naked eye.
But whose eye? Do bacteria have eyes too?
Hmmm…
Okay, let’s stop talking about bacteria eyes here.
What about angel eyes? Angels have eyes, right?
Angels are excluded in this discussion!… Okay…
That latest definition is clearer than the previous ones, I believe.
An animal is
any living thing that can move and can be seen with the naked eye.
So we have to change things a bit here. Bacteria and viruses are not animals now. Let’s call them the invisible-animals… I mean, not animals… the invisibles… invisible-tiny-things.
Insects will be classified as animals, by that definition.
Angels are… well… not animals.
(Why do you keep mentioning angels!)
We, humans, by that definition, are animals too!
Alright! Good!
But… Wait…
Are we, humans, the final authority for defining things?
We want to think that we are, of course! But are we?
But who else is there? We are the only ones in here. So we have to define things for ourselves!
But… are we the only ones in here?
(Hint: The Christian worldview says that we know that we are not the final authority for defining things, and that we know that we are not the only ones in here, and that we are doing our best to suppress that knowledge. Scary right!? But the good thing is that it helps us to be ready about the future.)
(Future?)
