A thing (or phenomenon) itself is not necessarily, and perhaps simply is not, entirely and completely knowable.
The scientific mindset understands this limitation. The scientific mindset has a tentative relationship with knowledge.
The most organized knowledge about a thing is in the form of description we call theory.
We say that theories provide models of various types (analogy, mathematical, and others) that help us have a sense of understanding a thing.
Scientific theories themselves are models of the grandest type.
The thing, the reality, is not completely accessible to us, or even if it is, we can not be certain that we are completely accessing it, as we do not know what we do not know.
Our understanding of the thing is, for practicality, our understanding of our model of the thing.