Once when Shri Nathji was confronted by western philosophers and he was asked to prove the existence of God, he replied:
The Sun can only be seen with the light of the Sun. You cannot light a candle to reveal the Sun.
He then, asked them: There is wine in a cup. Can you prove the existence of intoxication in the wine? No. Then how can you prove the existence of intoxication?
He answered the question himself: The existence of intoxication can only be proved in the person who has drunk the wine. It is a strange thing–the wine is no more visible, but intoxication begins to manifest itself in the words and gestures of the one who has drunk the wine. His entire personality is a proof of the intoxication within him. Similarly, the existence of God can be felt in that person whose personality and whose words and actions, give ample manifestation of divine qualities. It is difficult to actually prove the existence of God in the Universe, but His Real Self can be seen and felt in the personality of the avatar.