Nothing can truly be known to exist.
Müeller-Lyer illusion
Ponzo illusion
Both sets of lines in the pictures above are the same length, though they do not appear to be. It certainly calls into question the reliability of the senses with which we perceive and judge the world. So on what basis can we give our senses any credibility or authority?
A ruler can show that the lines are the same length. Comparative analysis seems to be a good way to corroborate similarity and congruence but does it prove existence? Not necessarily. Is there an equivalent ruler with which I can measure myself in order to discern that I am indeed a human being like all other human beings? The consistency of our senses, the fact that the 'true word' exists (whereas dreams and hallucinations pass and make evident their falsehood) in one continuous and unbroken thread of time makes room for the leap of faith that we take when we say that we exist.
Until my existence drifts away like a dream or fades like a hallucinogenic trip and I am thrust into a new reality, I must say that I and the things around me exist. Of course it remains a leap of faith, but without proof that anything exists or that, more importantly, nothing exists, a certain acceptance is required to live on through the world, for if nothing exists, how do you explain the world around us?