Is "thing" the most general in meaning

Solution 1:

Thing, being, entity, substance are all words that can be used as the terms for what there is, when one doesn’t want to say anything about it other than that it is.

Generally, the first three imply that we are referring to something countable; substance is usually used for something uncountable. That, however, is not a strict rule. If I am asked whether there are any white things over there, it wouldn’t be wrong to respond ‘yes, there is some snow there’. On the other hand, substance was used as a countable term in earlier periods of the history of philosophy (and consequently may be so used in the present-day discussions of the works from that era).

The meaning of substance, incidentally, does not restrict it to material substance; it is possible to speak of spiritual substance or mental substance. One may, of course, claim that material substance is the only kind that there is, but that is a matter of one’s philosophical theory; the meaning of the word substance does not by itself rule out the other kinds.

Setting aside substance, it is largely a matter of preference and style whether to use thing, being or entity. The last one belongs to a more formal register, but is otherwise interchangeable with the other two.

In everyday English, thing is often understood to imply that we are speaking of something inanimate, but that is also not a strict rule. If I am asked whether there any white things over there, it would, again, be natural to include polar bears among the white things.

One should bear in mind that although these four terms are highly general, they are usually (but, again, not always) understood to stand in contrast to qualities, properties, characteristics, etc.: things have properties.