you said the only thing we can ever know to be concrete in our life are our 5 senses...
so basically everything outside of our consciousness or in other words, are mind, would be known as the external world, and everything in the external world you believe to be true, because you have perceived them as real, using your 5 senses. Lets use a rock as a object from the external world. if we are physically holding this rock, we use are using our senses to make us believe that the rock exists....therefor everything we believe to exist in our lives is all solely dependent on our sensory receptors, that allow us to taste, smell, see, touch, and hear everything in our life that we've experienced.
Now lets say that a evil scientist was implanting electrical probes into our brains. This scientist would play on a keyboard that the electrical probes were connected too, this would cause electrical impulses to be sent into are brain, which would control everything in our mind. Having complete control of our mind would allow this scientist to control our "five senses", which would give us a false perception of everything in our lives we believed to be complete truth. so the rock we were previously holding would now have a possibility of not existing and being merely a false perception in our mind, because there is a possibility of a evil scientist making us think we are holding this rock....this idea is similar to the concept of the matrix
now being told that everything you believe to be true and exist in the external would has the possibility of all being a delusion, would bring discomfort and trouble many. Sadly however, since there is a possibility of everything external to our minds being all a false perception, proves that nobody can ever fully know for certain anything regarding the external world. So in order to have knowledge for certainty regarding anything in external , one first must prove the that there is no evil scientist falsely perceiving you into doing so
so in conclusion,
-since there is no way for anyone to prove that the evil scientist falsely perceiving us does not exist
-then there is no way for anyone to ever achieve knowledge of anything regarding the external world