Can we ever have "true" knowledge?

How do you define “knowledge” and why? In philosophy, the generally accepted definition is “true and justified belief”. What do you believe in that is both true and justified by fact? What can you say, for certain?

I, personally, have had a lot of trouble explaining the existence of anything but myself. In fact, while I love others and enjoy their company, I can’t PROVE the existence of anyone but myself. Therefore… Can I ever truly KNOW anyone but myself? Please, feel free to lend me your insight. And yes, those of you well studied in philosophy will recognize that lately I have been reading both Descartes and Berkeley.