From the time of our birth, our senses have fed our brains with a constant stream of electrical impulses.
Our brains look for reoccurring patterns in that stream, comparing it with past input and assembles larger and more useful blocks of information as to what those impulses represent.
For example, a baby is thought to only see light and dark at first but soon is able to recognize its mother's face.
It's amazing that our brains are prewired to do this sort of pattern recognition at all.
The problem is that that is all we will ever know about the outside world. Just the patterns the brain thinks it is recognizing in the incoming stream from our senses.
Think of a thermostat. It has only one sense, temperature. If there were a loud sound or bright light or peculiar smell, the thermostat knows nothing about it since it has no sense to detect it.
We know from magic tricks and optical illusions that the brain can easily be fooled.
But that's all we have.
Unfortunately, that's all we will ever have.
We have only brain created images of what is out there and there is no way to know if they are real or accurate in any absolute sense.
We will never know for sure if the blue sky that I see looks at all like the the blue sky you see.
Science has developed tools to extend our senses. Microscopes allow us to see the very small, telescopes to see the very distant. But ultimately information has to come into our brains through our senses and be interpreted by our brains as to their meaning.
We should not assume the physical world, as we see it, is all there is.
One hundred and fifty years ago we couldn't detect the presence radio waves or radioactivity, yet we know now we have always been surrounded by them.
It is very likely there is much more out there than we will ever know. Most likely a lot more.
"There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy."
Shakespeare - Hamlet (1.5.167-8), Hamlet to Horatio