Here is a hand

Philosopher G. E. Moore's common-sense argument for the existence of an external world. Goes like this:

1. Here is a hand.
2. Here is another hand.
3. There are at least two external objects. (From 1 & 2)
4. Therefore an external world exists.

Moore briefly presented the argument in his Proof of an External World:

I can prove now, for instance, that two human hands exist. How? By holding up my two hands, and saying, as I make a certain gesture with the right hand, 'Here is one hand', and adding, as I make a certain gesture with the left, 'and here is another'.

For a discussion of the argument in greater depth, see user Gritchka's write-up in the G.E. Moore node. To learn what some other philosophers had to say about it, check out frankdeluxe's Wittgenstein, Ayer, and certainty.