In natural language, the present tense of the verb ``to be'' is used for asserting an intrinsic property of an entity and for asserting a property that is expected to hold long enough to provide a constant background for other events under discussion.
The wealth or welfare of a human or animal is such a temporary property.
The welfare of a mosquito over a short time is definable. It harms the mosquito to be squashed and helps it if it finds exposed skin from which to extract blood. Over a year the welfare of an individual mosquito is not definable. If it is to be defined, the concepts, e.g. descendants, will be quite different.
This suggest using contexts. We have
as a proposition about this particular mosquito.
A person's wealth at a given time can be measured as an amount of money. His wealth increases as he is paid and decreases as he spends money. However, over a period of 10,000 years, the wealth or welfare of this individual is undefined.
Nevertheless, wealth and welfare are useful concepts.
Fiction provides an interesting class of approximate objects and theories, especially historical fiction, in which the author tries to fit his characters and their lives into a background of historical fact. Common sense knowledge tells us that Sherlock Holmes would have had a mother, but Conan Doyle does not provide us with a name. A definite address is given, but there was never a house there corresponding to Doyle's description. The author need only define his world to a point that lets the reader answer the questions the author wants the reader to ask.