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.