"Abstract object"? You're in trouble coming through the door. Most decent questions are traps for the unwary - briar patches, if you will. One way to avoid entanglement is to start with some fairly careful thinking about your subject, probably starting with definitions. These steps alone take care of most problems. To be sure they can also squeeze some of the fun out.

I think you have to answer your own question, then see if your answer works. Likely you will have to determine what definitions you need, to start. Don't be put off if available definitions don't work and you have to supply your own. Even if it turns out you're wrong it will still sharpen your thinking. And of course if you are wrong, all of us here at TPF are here to let you know!

I am sometimes wondering whether stock price indexes such as Dow Jones Industrial Average, NASDAQ Composite, etc, are existing objects or not. If they are existing things, are they concrete or abstract??? — A Son of Rosenthal

This is an Excellent Question. Let's not go into too much depth with it, lest we stumble upon some highly inconvenient truth. The price of a stock is supposed to have something to do with concrete reality, right? But we know that the up and down movement of stock prices is often driven by fairly whimsical, non-production events. (Like, "After Angela Merkel slapped Donald Trump's face and said he was "Zu dumm fur Worte" the DJIA dropped 259 points.")

It seems like some stock take on lives of their own as speculative instruments. Real money changes hands, the stated value goes up and down, but to what is that activity tied? It's hard to tell. At least the stock of some companies is tied to real activity. Electrolux actually produces things (lots of appliances). BNSF is a real railroad that hauls around real stuff like waste paper for recycling, coal, new cars from Japan, and tea from China.

Money itself (fiat currency) is pretty much an abstraction; it's based on faith. Insubstantial. We hope to God that the faith in our money lasts. If not, we will be maximally screwed, and the screwing won't be at all abstract.

Money itself (fiat currency) is pretty much an abstraction; it's based on faith. Insubstantial. We hope to God that the faith in our money lasts. If not, we will be maximally screwed, and the screwing won't be at all abstract.

We as a nation confer certain values on currency, we give it a social reality, a social function, social power. It's has a subjective ontological reality. A stock index is a measure whose reality is derivative of the corporations it tracks for value.

Succinct summary. I hope it keeps working.