You know, this argument falls a bit flat for me, because we mostly have no idea how we are doing it, either.
The problem with defining inteligence is that we only know symptoms, but have only superficial knowledge of the mechanics. It's almost as if we say that "computers can't really be intelligent, because we know exactly, in the tiniest detail, how they arrive at their conclusions"*, while we mythologise our own thought processes because we don't understand them yet. Ergo, we say "intelligence should be able to do this, this and this, but if it achieves this ability by these or these means, it doesn't count!"
*not entirely the case anymore with complex neural nets like DeepMind, that is very much producing results people weren't expecting, and they're now trying to find out how it does it, despite that they have written it and can look at every line of code... At one point, interactions become so complex that you get an emergent system simply in the sense that you cannot predict how the individual parts will interact. And our own intelligence might not be too different from that.