Your observation of the reading on the scale is true, of course. Your observation that the weight is 51 grams is false.
"This weight masses 51 grams" is not an observation, it's a theory attempting to explain an observation. It just seems so immediate, so obvious and inarguable, that it feels like an observation.
I feel this leads into a rabbit hole where everything beyond photons striking the retina becomes a "theory".
Another month, another rationality quotes thread. The rules are: