If we say with Thales, '_Water is the source of all things_,' 'water' seems to be used collectively.

But substantive names are frequently used as general terms.

For example, _Gold is heavy_ means 'in comparison with other things,' such as water.

And, plainly, it does not mean that the aggregate of gold is heavier than the aggregate of water, but only that its specific gravity is greater; that is, bulk for bulk, any piece of gold is heavier than water.
Finally, any class-name may be used collectively if we wish to assert something of the things denoted by it, not distributively but altogether, as that _Sheep are more numerous than wolves_..