“Things on a very small scale [like electrons] behave like nothing that you have any direct experience about.
They do not behave like waves, they do not behave like particles, they do not behave like clouds,
or billiard balls,
or weights on springs, or like anything that you have ever seen.”
Richard Feynman
Probably in early history what defined a thing was the non-thing known as “air”—the tree is different from the mountain because there is air between them. It took thousands of years before science figured out that air was also a thing (even though the Greeks suggested it).
A thing can be material such as “car”, “table”, “mountain,” it may be m ore independent such as “body” or part of something like “hand,” it may be conceptual such as happiness, and a thing such as a smile or intelligence or beauty is in the eyes of the beholder.
This is a collecton of posters where each of the pictures share something in common, they are same things. At the same time on any poster you can see how different each thing is from the other.
Besides thinking things are Real we like to beieve that mathematics is Real. 1 +1 = 2, Yes! The thing is in this universe no two things are the same—in this sense no things are equal. But that is not a very practical way to live.
Mathematics requires the concept of two or more things, or a class Isn’t a thing a thing? A rose is a rose. we do it all the time.. Sound confusing? Not really.
Apples for sale at twenty five cents a pound or three for a dollar. At three for a dollar bigger apples are class different from the class of smaller ones. At per pound the quality of the apples regardless of size is a class. We have a traditional saying about mixing apples and oranges when we encounter poeple doing this.
How about the present day discussion of Pluto. Sometimes we classify something and then later decide (find?) it is no longer part of that class.