cf. an existential argument (I believe by Sartre), which claims that there can be no proof (scientific or otherwise) for direct communication between people and God:
http://www.tfproject.org/tfp/showthread.php?t=49226
To summarize:
Let's say one night as you are sleeping and suddenly you are woken up by a bright light. You see at the foot of your bed an angel who holds a burning sword. He tells you that God has sent him to tell you to live in a cave for the next five years of your life. The angel refuses to tell you why it's so important. What do you do?
The point: most people would not pack up their things and go live in a cave--they would think that they ate too much pizza the night before and had a dream, or, if it's a recurring dream, that they need to get medication or see a psychologist. The question is, at what point can you be convinced that the angel is really a messenger of God? What proof of authenticity is needed to convince you that the angel is a messenger of God? Some philosopher used this as a basis to say that God could never give a strong enough proof that such and such was what God wanted him to do. For all purported messages from God to him (or to the general populace), and for any conceivable hypothetical message, there always seems to be a more plausible argument about how that message came to be. Therefore, it's better to create a direction of life, a way of living, independently of any expectation of what God wants you to do. So goes the argument.