That is not true. We can detect the presence of a thing by the observation of something it causes. That does not imply we have a good definition of the thing.

At that point, we can only define it as something that causes this observation. And that is not very useful.

yeah! Just like gravity, which we all know is, uh, umm, uh, hold on a second,

Wait, I meant light, yeah, photons! It's photons all the way down! And what are photons you ask? shit, no more questions. Got to go.

https://en.wikipedia.org/wiki/Set_(mathematics)