Because of the living entity's ignorance, his senses are engaged with material objects. Since the living entity is alive, he can understand the creation of this material world to some extent, but he cannot understand the Supreme Personality of Godhead, who is beyond the conception of the body, mind and intelligence. Nevertheless, great sages who are always in meditation can see the personal form of the Lord within their hearts.
"Since an ordinary living being is materially contaminated, his words and intelligence are also material. Therefore he cannot ascertain the Supreme Personality of Godhead by manipulating his material senses.