My French teacher at school was prone to wandering off the subject. I don't know if that was why I got such poor grade at French O-Level (in truth I've never been much of linguist anyway), but I am still grateful for Mr Fallon for introducing me to Descartes' Cogito Ergo Sum.
You can argue about whether it really proves that 'I' exist, or what it means to say I exist (what do I mean by 'I', what do I mean by 'proves', what do I mean by 'exist'?) but the basic idea seems pretty undeniable. The greatest claim to reality, for me, is my passing thought. (And for you - if you exist - it's your passing thought.)
Where you go with that, of course, what use it is, is a different question.
To go beyond the 'I', requires an act of faith. You take the next step on trust. But what is the next step, what is the first thing outside that one thought? The next step is that there is something other than my thought, that there is something that my thought connects to. To say what that is would require a further step. To say that there is really a chair that I'm sitting on, really air that I breath, really a computer that I'm typing on, is an extra step of faith. All these could be illusions, dreams, or virtually reality. This, of course, takes us into the territory of The Matrix.
Again, though: so what? Well, for me, that 'next step' from my passing thought to something more, takes me to information. It's just that if we search for reality, we start with Cogito, move out to information, and only then can we acknowledge the reality of the material world.
The thing is that I suggest that this the opposite way around to received wisdom. Matter is supposed to be really real, information is a bit iffy, and the subjective stuff inside my head, well, that just can't be trusted.
In his talk at Turing 12, Luciano Floridi made a passing comment, something about young students getting over-enthusiastic about information, that there is nothing but information. He, Luciano, was cautioning against going too far. Maybe, but at the very least I think information demands a better hearing. I think the tyranny of materialism has had its day!
[Added later. I suppose this is wandering towards existentialism.]