Since all sensory data is mediated by our perceptual system we are in a similar position to a blind man with a stick.This is Kant's point.
The rest of your comment fails to take into account that 'similarity' and 'difference' are contextually functional
and are not 'properties possessed by a 'thing' per se
. In what way is yesterdays 'tree' the same
as todays 'tree'? Certainly not the molecular level ! As already dicussed 'thinghood' is assigned by communicating humans for joint purposes. Consider the 'thing we call global warming', and the disputes about the functionality of he term. Or consider what we call a single substance 'water' which some cultures have two different words for according to whether you were allowed to drink it ot not.And since all is in a state of change, 'thinghood' boils down to a prediction of an expected contextual interaction with the world relative to human lifespans. Heideggar was one philosopher who expanded on this with his description of 'things at hand'. Other philoophers interested in 'being' or 'what is' attempted to ban the verb 'is' since it implied existence independent of an observer.
As I have stated before, all this is about whther you consider 'existence' to be relative or absolute and religionists tend to go for the latter.