I think what you're saying is correct when it comes to perception with our senses, the kind of knowledge we share with the other higher animals. However, there is a higher level of knowledge that is unique to humans, which is rational in nature and can come up with certain knowledge. We can know certain mathematical and logical truths with certainty, such as 2 + 2 = 4 and that A and not A cannot both be true. We may not get an accurate count of cows in the field because one is hidden from view, but we can know with certainty that if we have X cows in field A and Y cows in field B, we have a total of X + Y cows in the two fields.
I agree with your main points, but I think it's worth pointing out that even things like 2 + 2 = 4 and X + Y cows in fields A and B, all rest on certain definitions and sets of deductions, many of which go back down to basic axioms which have to be assumed (e.g. for arithmetic, see Peano axioms as an example: https://en.wikipedia.org/wiki/Peano_axioms).
- We have a definition of what a cow is, and we know that cows are discrete/physical objects, and have relatively fixed locations (i.e. that they are not like an electron cloud with a probabilistic location).
- We assume that fields A and B in your hypothetical have clear, non-overlapping boundaries.
- We assume that we are working in a fairly normal universe with a fairly standard model of physics, and that due to the way time works in this universe, a cow cannot simultaneously be located in both fields A and B.
- ...
- (this could get really pedantic and go on forever)
The point is, even the things "we can know with certainty", are only as certain as the framework of observations/deductions/axioms/etc. that they rest upon. Almost nothing is certain on its own, without any further layers of reasoning behind it.
