Think of something that is unrelated to anything else.
For example, think of a separate universe from our own; call it UniverseBeta.
UniverseBeta has an internal measurement of 10 Blipblips. Now, if anyone asks you can confidently tell them that UniverseBeta has an internal measurement of 10 Blipblips.
What is a Blipblip? Is it a distance? a temperature? some quality that doesn't exist in our universe?
Did you know that an electron has 22.6 Quangflops. Quangflops have no effect on the behaviour of electrons, but electrons have 22.6 of them.
Suppose I want to check that the value of 22.6 is correct for a given electron. How would one go about measuring that value? What observation would confirm or deny that value given that the value doesn't change any observation in any way?
The reason we assign qualities and values to things is to explain their behaviour. Their 'behaviour' is just another name for their 'observed relationship with other things over time'.
A quality and/or value that doesn't impact behaviour is a moo point ( "Yeah, it's like a cow's opinion. It just doesn't matter. It's moo.").
An electron has a charge of -1 (or 1.60217662 × 10-19 coulombs)
How do we know an electron has a charge of -1? Because we measured it.
How did we measure the charge of an electron? By observing the behaviour of an electron relative to other things.
So... we've never actually seen a charge on an electron - we've seen a behaviour and interpreted that behaviour as being due to a charge on an electron.
This applies to every fundamental quality. We have only ever seen behaviours and created stories to explain those behaviours. Some of those stories are so successful at explaining the behaviours that we observe that it seems positively churlish to draw a line between the story and the presumed underlying reality.