How do we know things exist?

Are we aware of our surroundings? Simple things and day-to-day tasks are the things that we
are aware of, such as our physical future and our natural surroundings. However, how do we
exactly know if it exists? What if everything was just a dream or when we make up we feel it is
was all a hallucination. So trying to prove that there was an outside world would be a part of the
dream. Trying a different way to prove that there is an outside world would be useless. Slapping
or pinching yourself to wake up from that dream would not work.
Observations can be mistaken, but other observations can also correct it. Our idea of things that
exist is just our idea. If we see a snake in our dream and ounce, we wake up; we will realize that
it was just a reflection of a grass. Our belief in the world is outside our minds comes very
quickly; therefore, we do not need any ground or proof for it. We can live with it because there
may be a possibility of our belief becoming false, or we have no basis for ruling out the
possibility.