No! You can't. Over time several attempts have been made, but they are all rather pathetic and inadequate.

I shall therefore not make another hopeless attempt; let a small hint suffice:

What do we know with certainty? Do I know for sure that the computer that I'm currently working with really exists? No. Perhaps it's just a hallucination. In fact, everything around me may simply be hallucinations. Maybe the so-called "real" world doesn't exist at all. I see, hear, feel, smell, and taste things, but perhaps these senses really only exist in my mind – a kind of advanced dream.

Basically, I don't know with 100 per cent certainty that the world around me exists. It is, after all, a question about whether I believe my senses or not. If I am sufficiently stubborn, I could feel justified in denying that the physical world exists.

But there is one thing whose existence I can't deny: My consciousness. It is possible that the "real world" around me merely is a hallucination, but at least something exists that experiences these hallucinations: The abstract concept which we call "consciousness".

So to summarize: The only thing I know for sure is something metaphysical, namely that my consciousness exists. Whether or not the physical world exists as well, is a matter of faith.

With this in mind, one is less inclined to deny everything that lies outside the physical world, and one is not so cocksure that things that can be measured and weighed are especially important.