There are varying definitions of knowledge. What are the criteria for us to know something?

René Descartes (the father of modern philosophy) suggested that knowledge (now known as 'Cartesian knowledge') must fit the following requirements:
1) One must believe it
2) It must be true (it must be reality)
3) There is no reason to doubt otherwise

Naturally, Descartes has ushered in a movement called Solipsism almost by accident, which simply believes that one cannot know if other minds are actually conscious. For example, I know I am a thinking person. But how do I know you are? I must believe it, it must be true, and I should have no reason to doubt it. But I have at least one reason to doubt if you are a thinking being, so I cannot know (in the strict sense) that you are a thinking being like myself.

I'd like to hear everyone's definitions of knowledge, and some good healthy critiques will be highly appreciated from peers.

[Disclaimer: most people claim to know something when they may be wrong.]

I used to think that everyone went out of existence when they left...But I was a kid then.

Knowledge...The Bible says a lot. What did Proverbs say? Something about wisdom and knowledge being similar or something... But knowledge and understanding are different.

I'm probably my twin sister's imaginary friend. Who can know?

We are all the center of our own universe (not that we are the most important thing, but that our perceptions naturally radiate out from how they impact us personally). In the immortal words of Buccaroo Bonzai "No matter where you go, there you are".