Look at the following two sentences.
Example sentence 1: The dog is an animal(A dog is an animal).
Example sentence 2: the water is a liquid
Example sentence 1] has two meanings, because the word "definite" is cognitive, and cognition is affected by a variety of factors, so it is natural to have multiple meanings.
The first meaning is to identify a dog from multiple dogs, such as a dog in front of you by pointing to it, or a dog mentioned in the previous context. In short, this is specific to a dog.
The second meaning is to define the dog class from the group of common animals, for example, when the context of the dialogue mentions animals, people will think of a familiar list, and the dog is a definite element from this list. In this interpretation, the dog is not a dog, but a dog.
The second meaning is called generic usage.
Obviously, the dog is ambiguous, and the first meaning will pop out of the mind first without special information, so the second meaning is only used in a limited context. The more common expression that people have is:
Example sentence 3] dogs are an animal(A dog is an animal).
No, for [example sentence 2].
Example sentence 4 】waters are a liquid
This is because water is uncountable.
Look back at [Example sentence 2]. If you point your finger at a glass of water, then [Example sentence 2] is no problem. This is the first meaning usage mentioned above.
Can the water be used in a second way? For example, when we think of water, we also think of cognizing water from it. The answer is no. This is because the word "object" requires the noun to be "object", and the "object sense" of water is very poor. If the container is different, the shape of the water body will change; Two bodies of water are mixed together to form a body of water), so you can't add the word "object" (e.g., a water can't either).
Although it can be cognitively determined like an animal set, water is not an object, so [Example Sentence 2] cannot be interpreted in the second sense. The realization of generic usage in the second sense is to remove the object word, ie.
Example sentence 5】water is a liquid(Water is a liquid.) ）
If the addition of water in [example sentence 5] becomes [example sentence 2], then native English speakers will only think of specific cognizant water bodies.
Of course, it is also because the sense of objects in water is weak (there are no boundaries) that it is uncountable (people have no sense of how many two three many waters are, and the meaning of communication is lost). This is the reason why [Example Sentence 4] is wrong.