This is similar to my previous question, here: An argument that everything is possible. I have an argument that everything exists. For, that which does not exist is not a thing at all. For example, the term "unicorn", is merely a term. It is just a word that does not refer to anything. So, my question is, has any other philosopher made this argument? And is it a good argument?
I recommend distinguishing between object and word (noun): on the one hand, objects exist; they are real. On the other hand, some words refer to objects while other words do not. Hence all objects exist by definition, i.e. everything exists. That's not philosophy, it's just correct language.
One may expand this consideration by introducing the term 'matter of fact'. Then some words refer to 'matters of fact' while others do not; they refer to fictional relations.
Only if you're Humpty Dumpty
"When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean—neither more nor less."
- a material object without life or consciousness;
- an inanimate object.
- some entity, object, or creature that is not or cannot be specifically designated or precisely described:
- anything that is or may become an object of thought:
- a fact, circumstance, or state of affairs:
- an action, deed, event, or performance:
You have redefined the word "thing" to cover only the first 3 meanings. You are specifically excluding meaning 4 which covers anything that can be imagined, whether it exists or not.
It is not a good argument, because it relies on you inventing a personal redefinition of a word in the English language, and furthermore that you are engaging in bad faith with anyone else when it comes to discussion because your personal redefinition is kept secret from them. The purpose of language is to convey meaning. Unless you preface any discussion with your new definition of the word "thing", then you cannot engage in rational discussion with anyone else.
And after that, you're back to the objections raised against your linked post. If you redefine "thing" to mean "entities which physically exist", then "'that which does not exist is not a thing at all'" is a trivial restatement of your redefinition, not any kind of logical argument.
Depends on your definition of "exists" (obviously; the different definitions of "existence" for numbers are well known) and the definition of "everything" which is not trivial at all (logical contradictions? non-constructive proofs of existence? only physical objects?).
Anyway, the closest argument to your question is probably the Level IV multiverse in Max Tegmark's book Our Mathematical Universe: My Quest for the Ultimate Nature of Reality. Short summary: if we accept that numbers (or generally, mathematical structures) exist, and the existence is independent on the raw substrate (the simulation argument), then our universe is not just described by, but is identical to a certain mathematical structure. By extension, any object described by a mathematical structure (i.e. by any reasonable, logically consistent description) exists. We can argue about the finer details, e.g. if we accept only computable or decidable or even finite structures, but that's about it.
I think a lot of answers miss that the question is actually interesting. It has been important in philosophy since its early beginnings.
Think about Plato's Sophist, which shows the argument between Parmenide's and Heraclite's schools, or Protagoras (as understood by Plato), through the voice of the Stranger from Elée. For Parmenide, what is is, and what is not is not. For Heraclite everything changes so nothing is ever what is it.
The answer Plato provides, accomplishing the so called parricide of Parmenide, is that even what is not is in a way. Something that could be called non-being, like nothing, something that is not in the world, something that is false, is already in a way, because we can think about it, talk about it, be influenced by it, believe it.
On the other hand, Plato doesn't flatten everything at all. An object that is in the world, is more, is really, compared to an object that is not. A speech that is true, is more than a speech that is false.
So he defends that middle ground, which is similar to what you say in a way, even if it is more complicated as you need to integrate all this with is ontological and cosmological system to really understand what he is saying.
Finally, this question is still discussed by contemporary fiction philosophers, as realists are saying : fiction characters exist, in a way, while others say that they don't exist.
You can also link this question to Aristotle's views on being, which is that being has multiple meanings, understood by many as a focal meaning of beeing : that which is is expressed in multiple fashion. See https://orb.binghamton.edu/cgi/viewcontent.cgi?article=1192&context=sagp
I believe that answers that give a yes or no answer are missing the point that this question has been at the center of philosophical debates since thousands of years and is still ongoing. If you just say yes or no, followed by an argument, you are bringing something to the debate, but you miss the point that the debate is ongoing in a way.
Phenomenology, realism, analytic philosophy do not state the same thing about the answer.
I am unaware of any philosopher presenting this argument but IMO this argument is perceived in both ways - good one and bad one. Allow me to elaborate with one small story -
In the time of the stone age, once there were two mothers talking. One was proud of her son because her son got selected for the hunting squad and she was telling the other mother of her son's gallantry and the other mother was complaining about her son saying that he spends his entire time in caves drawing paintings of their civilization.
The irony is, today we are able to figure out how people used to live in the stone age because of those paintings.
Coming back, today one of most useless tasks considered is teenagers posting useless videos on TikTok or YouTube. Now we all agree( and we should) that we don't know everything so there might be something good happening out of something stupid or bad also.
On other hand, sometimes I believe there is no meaning of creating new things or doing something. when we see something or do something new we help evolution process and pass our knowledge to future generation. And mind you, these are not depression thoughts or output of something negative but in all seriousness I believe sometimes there is no meaning. That Humankind shouldn't have evolved the way it have till now. Think about it, let's say we learn more and more, and able to contact aliens then what next? There is no way the population is going to decrease, scientist, innovators find ways to help humans and we have evil people blocking all this again. There is no way the evil is going to end. Even if we find ways to live on Mars, all these is going to continue on Mars also.