It's an interesting question. In order to know anything for certain, you must first be sure of your own existence - otherwise what you claim to know could simply be a by-product of the delusion that you exist (i.e. none of it could be true).
The classic argument for this, cogito ergo sum, was provided by Descartes:
I think (premise), therefore I am (conclusion).
A criticism of this is that Descartes is begging the question in his premise. That is, by stating 'I think' you are already assuming that you exist, and so you are assuming the conclusion from the outset. This seems like a valid criticism, but I would suggest that it is simply impossible to prove your own existence without begging the question.
I am exploring this possibility in my discussions on the Premier website forums. Sye TenB (a presuppositionalist - see here for background) claims that he is certain that God exists, and that he is also certain of x, y and z. My challenge to him is to provide his own proof that he exists for certain. To clarify, I'm not asking him to prove it to me, but just to provide the proof that he claims he has. He has so far refused to do so; instead he continues to try and shift the discussion back onto my 'worldview'. The reason that he does this is blatantly obvious - he cannot prove that he exists for certain without begging the question.
As I see it (and please correct me if there are more), the two possible proofs he has are:
1) Descartes' proof, which he doesn't accept because it begs the question
2) God has revealed to him that he exists in such a way that he can be certain. Ignoring the other issues with this statement, if he uses this proof, he is also begging the question as shown here...
P1: God can reveal things for certain
P2: God has revealed to me that I exist for certain
C: I exist for certain
The conclusion is being assumed by the use of the word me in P2.
Even though he refuses to critically examine his own worldview, if he is intellectually honest (which he claims to be), Sye is forced to concede that he cannot prove that he exists for certain. This negates any other claims to certainty as anything he thinks could simply be a part of the delusion that he exists.
Sye could accept that we all know we exist for certain, and then continue with his argument. But he can't do this because this would mean that certainty is possible without God - God is not needed for cogito ergo sum to hold.
So, do you exist for certain Sye? If so, please provide the proof.
[Note: Dawson Bethrick has posted a rebuttal of Sye's website here]
As can be seen in the comment section, Sye has declined my request to answer the question, but Dan Marvin has offered the following proof that he is certain he exists:
The purple weighs green therefore bathtub penguin the much
It is a very strange proof so I asked him several times if he was sure, and I also gave him the decision as to whether his proof had to conform to logic. I'll let the readers decide if they are convinced. But by Dan's OWN STANDARDS, here is my irrefutable counter-proof that both he and God don't exist:
P1: Mice when buckets you are a joke
P2: Week the go suck a lemon sunshine
C: Dan and God don't exist
I am going to keep a running list of each time Sye has evaded answering my simple request: "Do you exist for certain Sye? If so, please provide the proof." - (not necessarily in chronological order)
One, Two, Three, Four, Five, Six, Seven, Eight, Nine, Ten, Eleven, Twelve, Thirteen, Fourteen, Fifteen, Sixteen, Seventeen, Eighteen, Nineteen, Twenty